My clown fish is not doing good! I’ve had my tank for about two weeks now. Gill is swollen and have white spot! Note I have two clown fish and only one is sick. What should I treat my new tank with?

Question, I’m a beginner and don’t have a RO/DI machine yet. What should I top off my saltwater aquarium with water that has evaporated? Treated water where I add some drops of water conditioner or distilled water?