I have 800 gal 3' deep pond in Huntsville TX. I have put 250watt heater in it and foil faced urethane sheath covering 95% of the surface; a small filter is running for gas exchange. At noon Jan 7 it was 54F. I had seen some danios in it (swimming ) in mid-Dec. They are not going to like the next few days. I expect no problem with 5 each 24" Koi.