I've used it without adverse effect on fish and not too much effect on algae. I wonder if the different experiences might be due to dissolved substances in the water, either hardness or TDS. My water is rather hard, 55 GH, 12 KH, and about 1000 ppm TDS, If AlgaeFix does work by making the algae cell membrane leaky, then the higher osmotic tension of my water might make it less effective.
I've recently used it against chara in an outdoor container, the chara had a setback but returned as if nothing had happened. I need to try periodic repeat treatments, I guess.