My tank is 4 months old now. All fish have been doing well. Also all inverts and leathers too. My 1.5 inch long clown has been there since I started the tank. One day I did a partial water change and he was dead the next day. 1 week later the smaller clown was dead too. Used pure water & proper salt mix. All water parameters are optimal, all other life is doing great. WA HAPPEN???