New to Saltwater Tank

Hi, I am new to keeping a saltwater tank and have been having issues with fish. I have had a damsel pass and the other damsel in the tank seems to be coming up with the same thing so I took it out of the tank so it wouldn't (hopefully) spread whatever it was to my clownfish that are in the tank. I have no idea what the problem is as I have been testing my water and get good parameters and my clowns are very active and swim all over the tank (now it is just them.) Any help would be greatly appreciated.