I had the same problem and lost all my fish. I made some mistakes I learned from. If it's hot and your temperature swings up rapidly then that could cause it. Set your thermometer to 80 that way if it gets hot it won't be such a dramatic swing. "Ick attack" you can use on your tank but you have to turn off your skimmer and take the carbon out of your filter. It takes longer but it's supposed to work. I have a 10 gallon hospital tank now with just a heater, HOB
filter some live rock and a damselfish living in it. If you treat the sick fish with copper you probably won't lose your fish and won't contaminate your tank. Try " ice attack". I also bought a UV
sterilizer and attached it to my filter. It's supposed to help.