If that is a Fluval Evo I had a similar situation but caught it before I lost my clown. Those tanks are mass produced and my only guess is some chemical is used in the production. I didn't thoroughly clean my new tank before I set it up and almost lost the clowns. I took the clowns to the fish tore to hold, broke down the tank and thoroughly cleaned it and tossed that sand. After I did that I ran fresh water for a couple of days and carbon. I then started it back up with new sand, new bacteria and added my clown back after a day and all was fine. I have to assume because I couldn't come up with anything else that there was something in the tank or silicon from manufacturing that was still present when I didn't thoroughly clean my tank before setup.