What is inversion?

[Esther]

I think that's what it is called. Something to do with winter water temps and their variations etc.

If a person had a large "natural" pond in a country where it is consistantly 20 degrees below, would it be smart to try to circulate the bottom water upwards to try to keep the pond warmer? It just seemed that I read about it at one time and people were saying not to do it.

 I am trying to get information for someone else and don't know enough about this to even intelligently ask the question.

[Craig]

I'm having to reach way back in my memory for this one, so I could be somewhat inaccurate but....

For the most part cold water is more dense than warmer and so settles to the bottom of the water column.  But due to the unique structure of the water molecule, as I recall, at about 4 C (39 F), the molecular structure changes and water colder than 4 C actually becomes less dense and rises to the surface...that is why ice floats.  So in a deep enough pond, the lower layer of water should stay at a nearly constant 40 F.

That's how I remember it anyway.<g>

[EagleEye]

If a person had a large "natural" pond in a country where it is consistantly 20 degrees below, up north here, the only reason to cirulate the water, like by using a fountain or something similar, would be to keep a hole open to keep the fish alive. some of our local small lakes do this to prevent fish die-offs. the farm where I hunt has a natural, so to speak, large damned up pond, but it is fed by natural springs, so it always has open water somewhere due to the springs.
Steve

[Mikey]

Constantly twenty degrees below what?  Zero?   

[Esther]

OK Mikey, smarty pants. Yah O degrees. I wouldn't expect you to know about temps like that,  where you live. There are places that get that cold, you know.

[andrew davis]

With inversion, it is quite possible to have sub zero air temperatures of 15°f, water surface 32°f (freezing) ...put your arm two feet down in still water and it is still a toasty 50°f something... quite cosey for wintering fish and dormant tropical waterlily tubers

Creating any turbulence would be likely to create 'super cooling' conditions where you are sending freezing and sub zero temperatures where you really, really don't want them to go...

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html

[Craig]

Not to seem needlessly argumentative but....

There will be no "50f something" water.  The entire water volume must cool to just below 40F before the colder water will begin to rise.  At fifty something you would still need to feed the fish.

Circulation that moves the warmer water from the bottom and mixes it with the cooler surface water will likely give you a pond of uniform temperature at or near freezing thus removing the thermal "blanket" layer the fish need to survive, but won't supercool it in the technical sense that the water temp drops below freezing without ice crystals forming.

[Teresa]

Not only is Craig right on target about how this works, he's right on target about the temps (you might find the very bottom water at just above 40, but you will never, ever get to 50 unless you are in the ocean).  And you'd have to have quite a lot of water for inversion to happen tho the water on the bottom of a good sized garden pond is usually warmer in winter than that on the top.

As for circulating it to keep it all warm, that won't work.  Moving water freezes at a much lower temperature than still water (something like 28 degrees as I recall, but don't quote me on that, look it up).  That means that if you start moving the water around, it will more than likely all go below freezing instead of all getting warmer.

I certainly would NOT try to keep dormant tropical water lily tubers at the bottom of any garden pond unless you don't care if they stay dormant permanently.

[Mucky_Waters]

Ah, the old warm layer on the bottom of the pond debate, I have read and been involved in a few of these over in the GW forums.
The only thing I know for certain about all that is that there is some misconceptions floating around about the topic.
My own personal view is that it is good to circulate the water in a frozen pond in the winter, and yes, even water close to the bottom of the pond.
Here is why I have come to believe that. First of all, who says fish need to stay warm under the ice? Most fish need to be cold during their winter dormancy (I'm talking gold fish and koi), if they warm up too much  under the ice (for whatever reason) they will likely get sick and die. Of course you don't want them to freeze either, but in a large pond that isn't likely to happen. Most people agree, a uniform water temperature is always best under any conditions, winter or summer. Any time fish get exposed to any significant water temperature extremes it is hard on them, so if there was a warm/cold layer in your pond it would be likely your fish would be swimming from one layer to another and at risk of getting temperature stressed.
Another point is oxygen and ammonia. If there was a warm layer at the bottom of the pond where fish hang out it makes sense that that is where the oxygen levels would be depleted first and ammonia levels would rise highest, also not good for fish.
Usually when there is fish kill in a ponds or lakes, it is because of a lack of oxygen, NOT because the water was too cold at the bottom of the pond or lake where the fish were suppose to be hanging out.
I would like to give further evidence to support my view, but I 'm off to work now.
Have a great day everyone.

[andrew davis]

When there are fishkills on ponds it is usually a build up of toxic gases below sealed ice, it usually takes several days to become critical

Fishkills also occur where fish gills are choked by frozen crystals from super cooled water

You can tell if it is oxygen deprivation by the number of frogs being killed first under ice, those die first in numbers, you see their bodies under the ice, where ice seals over warmish water and catches them when they are still active and they cannot get to the surface to breath. they have quite a high oxygen requirement compared to fish

If you are mislead into thinking that water needs to be moved under ice, tie a thermometer to a string, toss it down deep and check it around dawn.

Compare it to readings on a still pond and you will soon realise moving cold water presents
a significant cooling risk

If the temps are going lower than 35°f down deep it is quite possible temps are headed at or below freezing when water is moving, fish are at risk of their gills choking with ice crystals and temperatures are going beyond the limit of koi carp in particular

It does help if the numbers of fish under ice are low density and a small area of ice is kept thawed daily to vent dissolved toxic gases...

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html

[Teresa]

Ah, the old warm layer on the bottom of the pond debate, I have read and been involved in a few of these over in the GW forums.
The only thing I know for certain about all that is that there is some misconceptions floating around about the topic.
My own personal view is that it is good to circulate the water in a frozen pond in the winter, and yes, even water close to the bottom of the pond.
Here is why I have come to believe that. First of all, who says fish need to stay warm under the ice? Most fish need to be cold during their winter dormancy (I'm talking gold fish and koi), if they warm up too much  under the ice (for whatever reason) they will likely get sick and die. Of course you don't want them to freeze either, but in a large pond that isn't likely to happen. Most people agree, a uniform water temperature is always best under any conditions, winter or summer. Any time fish get exposed to any significant water temperature extremes it is hard on them, so if there was a warm/cold layer in your pond it would be likely your fish would be swimming from one layer to another and at risk of getting temperature stressed.
Another point is oxygen and ammonia. If there was a warm layer at the bottom of the pond where fish hang out it makes sense that that is where the oxygen levels would be depleted first and ammonia levels would rise highest, also not good for fish.
Usually when there is fish kill in a ponds or lakes, it is because of a lack of oxygen, NOT because the water was too cold at the bottom of the pond or lake where the fish were suppose to be hanging out.
I would like to give further evidence to support my view, but I 'm off to work now.
Have a great day everyone.

Mucky -
First of all, most of our ponds are not big enough or deep enough to have an actual inversion. 

Second, the water at the bottom of the pond will never be more than a few degrees warmer, and so will never be 'warm'.  The fish will not be warm, will not come out of dormancy, will not get sick and die as the parasites/bacteria that might attack them and make them sick won't come out of dormancy till the water warms significantly in spring.  And they won't be exposed to dramatical differences in temperature because we are only talking a couple of degrees at best.

Third, the oxygen will not be depleted in the lower water because it is still quite cold and cold water holds oxygen much better than warm water does.  40 degree water is cold - if you don't believe me, stick your foot in it.  Toxic waste gasses will build up under an ice cover, and fish might die, but that doesn't mean there isn't also oxygen disolved in the water.

Lastly, it's a fact that moving water can be supercooled to lower than 32 degrees without forming ice crystals . . . . fish may not be so lucky if they are swimming in that same water.  Have you ever seen streams running thru snow covered mountains - some even with ice on the surface or edges?  The water in those streams is often below 30 degrees but not ice because it's moving too fast for the molecules to stick together.

[andrew davis]

Teresa,

You can go from 32°f to 50°f in two foot of water on a still pond, with air temps around 15°f... Been there, done that, several times. Counted the dead frogs, broke the ice, lifted the thermometer.

With continued days of sub zero weather the pond settles down to a routine of 40 something two feet down over time, cosey for most critters and tropical tubers, where arctic stuff tends to last no more than a week or so

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html

[Teresa]

Ok - last post on this subject and only so that people who are not familiar with the concept of a thermocline can understand what it is and how it works.   Please note that there will NEVER be water that is almost 20 degrees above freezing at the bottom of your pond in winter.  The difference in temperatures could only be that great if the water depth was in the hundreds of feet.

Important points:

- water is densest when it hits 39 degrees - in the summer, warmer water will sit on top of water that is 39 degrees - when the fall rolls around and all the water gets to that temp or below, the 39 degree water falls to the bottom and the water above gets colder and freezes into ice at 32 degrees.

- this does not happen in a garden pond as it is not deep enough.  The water at the bottom will be warmer (if the pond doesn't freeze solid), but not necessarily due to a thermocline and not necessarily as warm as 39 degrees and generally due to the soil's frost depth in your area.

- in the quote below, they are talking about lakes that are 30 meters (90 feet) deep


http://en.wikipedia.org/wiki/Thermocline

Thermoclines can also be observed in relatively shallow lakes. In colder climates, this leads to a phenomenon called turnover. During the summer, warm water, which is less dense, will sit on top of colder, denser water that sinks to the bottom, with a thermocline separating them. Because the warm water is also exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface. As winter approaches, the temperature of the surface water will also drop until it approaches 4 °C (39 °F), which is the temperature at which water is densest (remember, water expands as it freezes into ice - that expansion actually begins before the freezing point). 4 °C is, generally speaking, the temperature of the water below a thermocline. When the entire body of water is at or close to 4 °C, 'fall turnover' begins - the thermocline disappears, (or, to say a different way, it reaches the surface) and the water from the bottom of the lake can mix freely with the water from the top. This process is aided by wind or any other process that agitates the water.
As the temperature continues to drop, in those locations where it does, the water on the surface begins to get cold enough to freeze and the lake begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C.
Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single point to oscillate (usually as a form of seiche). Alternately the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.


And Finally . . . . . another quote:

Depending on the geographical location, the thermocline depth ranges from about 50m to 1000m.

[Mikey]

I've read some hot pond discussions before about rocked bottoms, roofing liner, chemicals etc., but never one about inversion.  Now see what you've done Esther?

[Johns]

For those with "farm ponds, "turnover" actually is a bit more common in warm weather, and can result in large fish kills.  See http://aquanic.org/publicat/state/il-in/faq/fishkil.htm

[Esther]

This brings back memories or the old days at AWGS. I once asked about Koi Clay and boy was I sorry.

[Jerry]

I think this was a good discussion.  it is fine to agree to disagree!
Why did I post this you ask?  Some members worry that they may have been terse and possibly offended someone.  I read all the posts and only saw reasonable people sharing opinions
Have fun!

[Rocmon]

   Please note that there will NEVER be water that is almost 20 degrees above freezing at the bottom of your pond in winter.  The difference in temperatures could only be that great if the water depth was in the hundreds of feet.

Depending on the geographical location, the thermocline depth ranges from about 50m to 1000m.

Never say never. The first five words in your quote should tell you that your pond water can be 20 degrees above freezing in winter—just depends on where you live...
Kalispell is a little different in winter than Guantanamo Bay.   

Sorry couldn't leave that one alone.
What I like about all these heated discussions is that everyone has an opinion, but no one really has scientific facts. Yes, even wikipedia isn't fact... but, but, but, I read it on the internet it must be true...
Guess that's why were all ponders

[frloplady]

Pond was lowered last fall to 4.5-5.5 feet deep to be able to drain the skimmer.  Pond is bermed.
The top 8-10 inches froze solid (except for where my DIY deicer was doing it's job quite well).  I had a remote thermometer on the 4.5 foot end and the low I ever saw was 37.8 or something like that.

Koi also do not go "dormant" in cold water.  Slow way down, yes, but are not dormant.  I had a very difficult time catching fish one year in February due to the pond level going down 4-6 inches a day.  After netting most of the ice off the pond (thankfully it was a very mild winter) I had to drain/pump the pond down to about 1.5 feet to catch these "dormant" fish.

[Mucky_Waters]

Well since everyone is quoting the internet I have listed a few links below to some explanations of what is the cause of dieing fish during winter fish kills in small lakes and ponds. (I hope some of them are scientifically based) If you read a few you'll see that lack of oxygen is the common denominator, not the water being too cold. If larger bodies of water can become oxygen depleted under the ice in the winter, then our ponds can become depleted of oxygen that much quicker. Locally I know of two small fishing lakes that the local department of environment has set up an aeration system they run in the winter so they don't lose their stocked fish (trout) during hard winters. The reason they say they run these costly aeration systems to keep the water oxygenated. The fish are dieing from lack of oxygen, not the cold, not toxic gas build up (although if there was any toxic gas build up and aeration system would help alleviate that too)
I know  it is a common belief that we keep holes in the ice to allow toxic gas to escape, but just as importantly (if not more importantly), we keep holes in the ice to allow fresh oxygen in. Most of us realize how important it is to keep all our pond water circulating for good oxygenation and gas exchange I just don't see how the opposite is suddenly true because the water is colder and there is a layer of ice forming on the surface. Yes cold water holds more oxygen, but if you read the links below you will see that in the winter it is common for small ponds and lakes to become starved of oxygen and that is what is killing the fish.
I have heard in the forums before people say circulating your water will super cool it, but I think true super cooling isn't possible in a pond. From my own experience I circulate my pond all winter and it freezes over pretty good, I use an external pump that sucks the water off the bottom of the pond, though the pump and back into the pond and directed up towards the surface to (help) keep a hole in surface. I did keep a wireless min/max thermometer sunk in the pond last winter and never had a reading below 37 F. My main pond is only about 650 gallons, only 30" deep and one side is above ground. The hole did at one point freeze solid and the ice thickness got to about 8+ inches.
In order for ice crystals to form on fish gills your pond I think you would have to be very small pond and the outside temperature would have to be very cold for a long period of time, so cold that you would no longer be able to keep a hole in the ice on the surface of the pond.
In the simplest of terms I think that once ice forms over the surface it acts as a sort of blanket barrier from the cold air above and the water is kept warm by the ground below which is good, the only problem is the ice also prevent the normal gas exchange of oxygen in, carbon dioxide out, which is bad.

Winter fish Kill link (1)
Winter fish Kill link (2)
Winter fish Kill link (3)
Winter fish Kill link (4)
Winter fish Kill link (5)
Winter fish Kill link (6)

I mean no disrespect to anyone in the forum, and I like to hear all the different opinions and ideas, and I'm sure willing to change mine if I can see enough logic and evidence to do so.

On a side note, one fellow in the GW forums monitored his water temps in one of his ponds last winter with 3 different thermometers at different depths. The top one at one point froze, the middle one measured temps similar to mine, the bottom one read temps in the 70 deg range. Why was the bottom one so high? His own explanation was that he did this as an experiment. The pond I think was about 6 ft deep, he deliberately left a few inches of debris on the bottom of the pond, he did not circulate the pond at all and made no attempt to keep a hole in the ice. there were a few fish in the pond, but they were not fish he was too concerned about. He believed that the debris in the bottom of the pond was composting and creating it's own heat, that's why the temp reading were so high. At one point he drilled a small hole in the ice and lowered another thermometer because he was wondering if the bottom thermometer was giving false readings, but the second thermometer read the same as the first. He said he was going to give an update in the forum in the spring as to how the fish faired, but I never did see it, perhaps I missed it.

[karen J]

I have kept thermometers in my pond for years, at different levels. The lowest temp ever recorded at the deepest level was 33.4, middle of Jan '06. Oddly enough, the dome/greenhouse I built last year was the best one yet, and I had very little ice on the surface (normal is 8-10").
If I had expendable fish, I'd try running my filter all winter. I used to do that when I had the submersible-type filtration, but I have a lot more to lose now.

I honestly don't believe that thermocline temp inversions are an issue with our smallish garden ponds. We have a lake management team in our town, and get annual reports- they post the winter temps at various water levels and yes there is quite a temp difference, but the lake is 350 acres and 60' deep.

[andrew davis]

Winter fish kills are rarely caused by oxygen depletion, under ice.

Let's look at what ice does, that is different to water that does not have ice... it seals over a pond. What builds up under ice, hydrogen sulphide (among other things). It is lighter than water, normally it floats to the surface and gases off.

This is a description of hydrogen sulphide, I don't think you need to be a scientist to realise what happens when you seal over a pond:

-------Highly toxic - may be fatal if inhaled. Inhalation of a single breath at a concentration of 1000 ppm ( 0.1% ) may cause coma. Corrosive when moist. Skin contact may cause burns. There is a rapid loss of sense of smell on exposure to gas concentrations above 150 ppm, and this means that the extent of exposure may be underestimated. Perception threshold ranges from 0.5 ppt to 0.1 ppm. Irritant. Asphyxiant.-------

That is just one of the goodies that decomposition in a pond makes, as a by product

Evidence that a single hole in a pond works, folk don't have significant fish kills where a pond is vented, venting works (no great science there) toxic gases, lighter than water, are released...

Now, a body of freshwater on the scale of a lake is different to a garden pond, by size and the scale of decomposition likely to be occurring (folk are more likely to keep the proportion lower on a fishpond)

But, you will see the principle of thermoclines, layered water in both... the risk of fishkills on a lake are likely to be more dramatic because of the greater scale of decomposition going on and the greater proportion of water with low oxygen levels, a lot more deeper water to potentially mix when inversion happens... so, while folk describe it as an 'oxygen' shortage the use of 'aerators' on lakes is quite likely mistaken... the chemical reaction of mixing low oxygen deep water plus inversion mixes toxic low oxygen water into the area where the fish are... under ice

It does not matter how much oxygen you pump under that ice, the likes of hydrogen sulphide is going to be sealed under and the fish have nowhere to hide

This is a different sort of fishkill compared to what a pondkeeper is likely to experience...

Most pondkeepers don't have such deep volumes of toxic water, when inversion happens there is not going to be so much toxic stuff being present under ice, but, it will accumulate...

But, there are several other threats likely to emerge caused by turbulence, using pumps or aerators through freezing conditions:

1) Pumps fail and drain a pond under the ice
2) Super cooled water, colder than freezing but still fluid are circulated into deeper water
3) The layer of ice is accelerated in thickness, a bit lethal where ponds are shallow...

If you really, really want to find out about inversion, first hand, go out there in sub zero conditions and put your arm in a still pond. The wimps may be permitted to drop a thermometer in instead

Assuming the organic debris is a modest low proportion, the pond has a modest vent, two feet down is quite a cosey place on a still pond, compared to a pond with freezing water moving around it... You can actually feel the difference in warmth in the frogs and the mud down there...

While your garden pond may have far less volume, it certainly will have that delicate inversion there through the winter months, sometimes you can see it on a winters morning, a speckly difference just six inches down, where cold brittle clear water is sitting over a gently warmed thermal currents below, with fine traces of clay like particles catching the light...

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html

[Johns]

One more time:  Want facts rather than opinions?.  Regarding pond, Inversion = turnover, or at least portends a turnover. I swear it does not appear anyone actually read the link I provided, but especially some did not bother to read the links Mucky provided, all of which are basically irrefutable sources from acadamia or government scientists. Please note that ALL credit lack of oxygen as the primary cause of fish kills in either warm or cold water.  Go back to Mucky's post or just look at this one that he included:.
 http://ohioline.osu.edu/a-fact/0008.html   First note that it is a "FACT SHEET". Once there click on your "edit" then "Find(on this page).. type "turnover" and click on "Find Next" and read about "premature fall turnover".  Therin lies a concise, authoritative, explanation of "turnover". 

For those of you who are computer capable, please accept my apologies for seeming patronizing, but there may be some who are unfamiliar with how to search an html document.

Glad to be of help.....

[Joyce]

John, maybe someone has been sniffing his own gas?    

[andrew davis]

Might the moderators consider removing the more banal comments from the obviously unprofessional and incompetent, instead of leaving them to trash a useful topical discussion

>>http://ohioline.osu.edu/a-fact/0008.html

This document features zero competence regarding major factors influencing water chemistry under ice. Roddy Conrad would probably laugh at such junk 'science'

You can have quite adequate oxygen under ice and the fish are all dead from the accumulation of hydrogen sulphide over time. Totally failed to address that.

There is a big difference confusing excessive high density fish stocks being killed by suffocation under ice by incompetent owners and that caused by the inevitable pollution that builds up beneath sealed ice.

Most fishkeepers do not have ponds deep enough to have low oxygen level problems, you have to go way beyond three feet deep to see water that anaerobic being turned over by inversion.

Both are quite separate preventable scenarios that some less than capable 'academic' has failed to differentiate

The obvious evidence is, why do folk who maintain an ice hole see vastly different outcomes on survival rates, compared to folk who leave ponds to ice over. You are not going to convince me that all those fish are getting enough oxygen from one measly melt hole.... uhuh, penny drops? so much for so called 'experts'

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html

[Craig]

With the exception of Teresa, this discussion has circumvented the original question in the post ‘What is inversion?’ and ignored the scientific fact that water is most dense at 4C.  This is every bit as ‘fixed’ a number as the 0C freezing point and 100C boiling for water.

The surface temp of any frozen pond has to be 0C or, more accurately, less than 0C as that is also the melting point of ice and water molecules at 0C are fluxing between solid and liquid at an equal rate.

To get to 0C, the water would necessarily have first been 4C and at that temp it would have sunk to the bottom.  Only when it had cooled further would the density change and it then rise to the surface and freeze.  This cannot happen if there is a layer of 50 something water between it and the surface.  Since the process obviously and irrefutably does occur, we can know that there is no water warmer than 4C in the pond.

Sticking ones arm in the water can give a deceptive and relative feeling of warmth as the hands and forearm are quite sensitive to temperature…a person running a fever of a mere 2 deg F will feel hot to the touch….but it can’t give an accurate idea of the temp, nor pass as a scientific approach.

[Mucky_Waters]

Esther originally had two questions, "What is inversion" and "would it be smart to try to circulate the bottom water upwards to try to keep the pond warmer?"
Although I didn't go into what an inversion was, I did try to explain why I think it would be a good idea to keep the water circulating even in the winter.
As for the what an inversion is? She hit upon the answerer herself in her post when she said "Something to do with winter water temps and their variations etc." In an attempt to explain how a pond inversion night occur, I can only relate it to what little I know about atmospheric inversions, and as Esther said it always has "something to do with temperature variations". Most of us have witnessed atmospheric inversion on a calm day when we have watched smoke rising from a chimney or other source and notice that the smoke will sometimes rise only to a certain point and then appear to hit a barrier, stop rising and begin traveling horizontally instead of continuing vertically upwards. This barrier is a level where there is a strong temperature differential (an inversion) that prevents the smoke from rising further as one might expect under more typical atmospheric conditions. Also something important to know with respect to all this is that air (or water) of similar temperature has a tendency to want to stick together and does not readily mix with air (or water) of dissimilar temperature, kind of like how oil and water don't like to mix. And so, even though as we all know, warm air (or water) rises,  under certain conditions a warm air (or water) layer can be trapped under a colder layer by an inversion and it's natural tendency to want to stick to together.
The mechanics and math of cool or warm air  rising and descending can get quite complicated, because as it rises or descends it's temperature also changes as the pressure changes. As air rises it expands and cools, and visa versa, as it descends it contracts and warms up. Of course water doesn't expand and contract as much as air, so there may be some big differences in the way air and water react in that way.

[Craig]

Sorry if I created confusion Mucky, but I never said it wasn't a two part question, just that part 1 had been circumvented.  And I don't find "Something to do with winter water temps and their variations etc." to be a very complete answer.  More over, when that part of the discussion ended, it did so with some misconception/misinformation given and that is what I was attempting to address in my post.

As for the atmospheric/water inversion analogy....apples and oranges.  Atmospheric inversions have a number of causes and vary in intensity.  In a freezing pond, there is one cause, the molecular structure of water and one outcome...the bottom layer of water will be 4 C and if the pond is frozen over, there can be no warmer water above it.  If for no other reason it would no longer meet the defintion of an inversion.<g>

[Roddy Conrad]

The main point to remember to avoid Winter fish kills is to keep oxygen in the water by preventing a complete freezover of the water surface.  If the pond is frozen over too long, all the oxygen will be consumed, and the fish will die.  This seldom happens in a natural lake because either they are too large to be completely frozen over, or there is so much water and so few fish that even if the pond or lake is frozen over, the oxygen is not used up fast enough to kill the fish.

Then there is the other point of how long the fish are how cold, particularly for koi.  Pond types of goldfish seem less sensitive to the cold water temperature than koi. 

For quite a few years, all I did for our outdoor koi pond was to keep a portion of the filter system running to make sure the water had some turnover through the air to keep oxygen content up, and biotreatment active.  But for a couple of winters, by the end of hard winter the koi were red streaked with stress and some of them were 'laying over" but not dead.  They all survived, but clearly they were on the edge of death. 

So made a DIY greenhouse in a day out of cheap PVC pipe, about $150 in pipe and fittings and plastic to drape over the pipe.  The last several years that has covered the pond and the koi did great through the winter at the warmer temperatures maintained by the DIY greenhouse.  Water temperature was typically 36 to 40 degrees F under the greenhouse versus 33 to 34 degrees F in the winters without the greenhouse.  With the greenhouse, I still keep a portion of the filter system running.  Early Spring the plastic is stripped off the PVC frame and then the frame is lifted off the pond for the season.

[Mucky_Waters]

Good advice Roddy.

Craig you could be right in all that stuff you said, I don't know. I did indicate that due to the fact that air can be compressed and expand a lot while water can not,  there would likely be some big differences in the way they react. I don't really know much at all about water temperature inversions. All I know is that it is good to circulate water in our ponds and I can't see having any dead (stagnate) areas with no circulation as being too good a thing.
As for atmospheric inversions I am all to familiar with their effect and how they inhibit the upward movement of air. Being a Hang Glider pilot for just over 30 years I have learned just how real a barrier an inversion can be between having a great cross country flight and a mediocre sleigh ride. For me the word INVERSION brings with it a lot of negative contentions.