It has an unpleasant smell, but is one of the best water treatments on the market. I use it all the time, I've only heard of one person having a problem with it, non-fish related. A friend of mine has an allergy to sulfa drugs, the sulphur smell is probably related to the hydromethane sufinate component, and caused him to break out in a rash. It did help with his problem of fry die off in his breeding set up due to crummy water conditions, and he keeps it on hand in case it happens again, figuring he'll deal with the rash.