ashfishy

Hi All, I have 5 compressiceps (2m 3f) that live in my 1200L with lots of other small tangs In the last few weeks 3 of them have been progressively sitting on the bottom more and more. They rest on their fins and breathe heavier than the other fish. When they do go for a swim it takes more effort than it should. I have done water changes and increased aeration but no change. The other 50 or so fish are fine. I think swim bladder issue? but why only the comps? and why not all the comps? Anyone got any ideas?