After expanding our pond this year we have had a huge algae problem and have been working on it via filtratio, chemicals, barley straw etc. the water never got any better and 70% is covered by floating plants. I have noticed a foul smell but chalked it up to sludge. Well when I got home last night my fan tail who is about 10 inches long was floating upside down but alive and would swim to the bottom when I turned him over, he would surface every few minutes swim upside down and then go back to bottom plus he has lost all his color. We decided that we were gonna drain the pond and start over we moved all the fish and put the sick one in a isolation tank. While draining we found 2 dead baby koi fish that I was unaware were in the pond they were about 1 1/2 inch long.

The water does smell like rotten fish now, could the dead babies make the water uninhabitable for the others and what could be making them sick? I do not want to lose my fantail but I have a feeling I will. Please help I do not want to lose the others