welcome! well it could be a million different things but we're here to help. First we need to know a little background info on your setup like BCSWEENEY said. You are talking about a salt water aquarium correct? Where did you get your tank? How long has it be setup? What type of filtration do you have? What kind of water are you using?
enjoy your stay here and dont worry, we'll help you figure it out.
I am a new hobbist My water test fine, but every fish I get dies. This last time they were thriving and even eating well I could even tell some growth. aaai bought some shrimp from Wally and I noticed the rapid breathing the change in the color.So I did awater change but slowly they dropped. One by one. The water smell like dead fish. Help!!!!!!!!!!!!!!!!!!