they are toxic, some less so than others. Its better safe than sorry when dealing with the lives of your tankI have to respectfully disagree on this. I've had two tiger tails crawl up in a crevice and die, and I had another black cucumber that got stressed and spewed some "super glue". None of these nuked the tank. In the first two instances the tanks were small 25-gallons and in the latter a 160-gallon. I'm not suggesting it can't happen, I'm just saying it isn't always a certainty.