Ok folks, I'm about to give up. My pond is in the middle of my house (think a square donut) so, even though outside I get little exposure to the predators. It is small at about 500-ish gallons and has just 3 Koi. The largest is about 12" with the smallest at about 8". I have done what I can to stop the growth of the algae but even after draining 3/4 of the volume, treating with an algaecide, adding beneficial bacteria, I can't seem to get control of this. No string algae just really, really green water. I'm considering putting the fish in a temp 50-60 gallon container and running a bleach solution through the system (post cleaning). It's either this or sending them off to the Otter Exposed pond at my friends house.. Any, All suggestions appreciated.