Yeah...if you are going to do saltwater, you pretty much have to test your water. If you have an LFS
, see if they will do it. You can't be sure otherwise. There is a post somewhere else with someone with extremely high phosphates. He is doing massive water changes, which you would assume would help, but they stay high after. It's not always about assuming in this hobby.
It could be high trates or phosphate. What water are you using? Could be a water quality issue. Could be something internal too. I've lost a fox face and a dwarf angel lately and am not sure about the cause. Chromis are fine though. My water (tested all the time) is fine.