A few years ago there was a warning around here (Wisconsin) about letting hunting dogs (or any dog) go in ponds with blur-green algae. I guess it killed a few dogs.
Yep blue green algae is not really algae.... Its a bacterial colony and a form of Cyanobacteria who happens to use photo synthesis Some of these (there are several) produce Neurotoxins.... Some are actually good for you.... But this kind grows in environments that are not even good for fish.
different species of it are very beneficial.... spirulina is one example
Oh by the way my water tanks have fish in them to graze on the algae on the walls and keep the Mosquito larvae gone. I dont feed the fish ever. Throw the gold fish (best able to survive huge range of temps) as teeny feeder fish... by the time I dump the tank to clean it out they are a good three inches.