I am not a chemist, but would have serious doubts it was caused by Amquel....at least not if the proper amounts etc were used. Amquel is probably one of the largest used products out there for coinditioninig water. I used to know what the main ingredient in it was but can't recall right now, but its not really a product that weill decompoose or go bad to the point that it will kill or injure whats in the tank. At most it would not lock up chlorine etc so its possible the fish got burned from chlorine ammonia, which I guess is a result of bad or out of date Amquel. Call em up and talk to them, I had issues one time with Hikari brand dechlor turning white or opaque from the usual clear color. Kaytee was real good about taking and running tests on the product I had and sending me the results etc, so I am pretty sure from what I hear about AMQUEL, they will do what is right if it proved it was faulty.....callem and see.