poison algae

[trout]

I need some help. Are there any algae that are toxic to fish. I am having fish deaths that I cant explain. 1 powder brown tang 5 days old eating well dead. this my be because of poisons used in his capture but today my Algae Blenny which was 5 months old died. I have a new Clown fish that ate well in the store now will hardly eat help. Any suggestions would be great. H2O is great 0 everything, 400 calcium. No measurable electrical current. I am at a loss and about ready to quit.