Three Dead Fish - What Do You Think?

[Julles]

Yesterday, as part of my weekly pond maintenance, I did a partial water change, as usual, 10%-20%.  I cleaned the filter media, and all was well.

This morning, I came outside to three dead fish - two small koi (4"-5") and one large sarasa, probably 10 years old.  Not a mark on a one of them.

Any ideas what happened?

All I can think is that possibly chlorine from the hose water when I added the new water.  But I do that every week and don't understand why that might hurt them this time.  Yes, we have had rain - do you think the city adds that much more chlorine after a rain?  We're talking 10%-20% water change in a 1000 gallon pond.

Also, I turn off the pump while I empty the pressurized canister filter and clean the media.  Do you think that the fresh chlorinated water entering one end of the pond, without circulation to move it through the whole pond, got too much concentration in one end, and the fish  were too dumb to move away from the chlorinated water?

Or, some other cause of death?

[Mike S.]

All I can offer is a few possibilities for you to consider.

From my own experience, I think that if low oxygen during the change was the problem, the larger Koi would have been effected first, before the smaller ones. But when a toxin is in the water, such as chlorine or chloramine, the smaller ones are usually more sensitive and affected sooner than the larger ones.

I have been told by people who run chlorination stations that the amount they add is varied with the local temps and rainfall levels. Houses closer to the treatment points will have higher levels of the treatment added show up in their taps. This more true when chlorine is the additive used, than with chloramine, as the latter is more chemically stable.

Two other thing to consider, but may not be all that likely due to the amount of water changed is the fact that water from the tap can have VERY low oxygen levels, and the pH can be quite different from the pond water.

An afterthought, water temp differences can also severely stress the fish, but once again, with a small water change, it would probably not be enough of a change to make a difference.

Wish I could offer something more concrete.

Mike S.
Spring Hill, FL

[Michiponder]

Another possibility could be that if you were treating for clorine and not cloramine.  Several years ago the local water company here made the switch without telling any one here and alot of fish keepers lost thier fish.