I have never used it but have never heard of it poisoning fish. However, from a holistic viewpoint, I think you need sunlight penetration for the creation of phytoplankton, the lowest order of the food chain. I can't see how you can have a balanced pond dyeing the water. Without sunlight penetration, I wouldn't think natural or chemical fertilization would work.
I believe Bill Cody addressed this matter once.
Bill?
