I know that some find me a bit "know it all" on some subjects, but the reason I have a strong opinion on so many subjects is because I've screwed up so many times over the years that I've "been there, done that" on a lot of stuff. As Grandpa Amos McCoy (Walter Brennan) used to say on The Real Mccoys, "No brag, just fact".
You can keep a 30 inch koi in 30 gallons of water if you have adequete aeration and biofiltering. BUT! If the power goes off, and it eventually will, you will have 20 pounds of DEAD fish.
Here's the deal: You need to stock your pond with an eye on the possibility of power failure. Now, if your filter system is keeping your water quality in good shape, your koi can usually survive 3 to 5 days with no biofiltering. BUT, (again) in a heavily stocked pond with no aeration, oxygen is soon depleted and the largest fish die first. Therefore, the key to keeping your koi alive even for several hours with no aeration, especially in hot weather, is maintaining a healthy relationship between the pounds of fish in your pond and the natural oxygenation via suface tension of the pond water. This is dependant on surface area, NOt gallons in the pond.
Interested???? See my webpage "FISH LOAD CALCULATOR" at
http://www.gardenendeavors.com/rack/web17.html . Key to successful calculation: A numeral MUST be entered for every fish size, even if zero.
BTW, this calculator has been on line since February, 1999, and I've never received any negative feedback on it.
It would be interesting if some of you would post the results for your ponds, wouldn't it????