There are toxic algae in ocean and lakes, but I am not aware of any algae in tank environment that is harmful to fish, but excessive algae growth can choke up aquatic plants. Algae can even be beneficial to fish by removing nitrogen from the water. But black beard algae is ugly, and blacken my rock and dotted my glass. They are hard to get rid of as plecos don't eat them, and have to be physically removed by hand. I read that only two fish will eat black beard algae, one is Flying Fox, and the other is Molly. I tried both but my cichlids ate them, so I had no chance to prove if it works or not. Despite the ugly look, one tiny solace is that BBA coated my larva rock making them less abrasive to fish.