Never had any problems with algae until a few weeks ago, We have had Koi and goldfish for 15 + years, our ponds total about 3000 gals, we researched this issue and from "The Pond Guy" website it recommended a couple of products, we tried one in a very small dose but the fish were gasping for air, we have 2 waterfalls, large pump and 2 filtration systems, anyone have any suggestions how to clear this up