New fish , old fish? Anyone else acting funny? Water parameters...Was the food completely thawed? Exterior signs of parasites? Choking on a large piece of food... Dropsy
It could be anything really.
When I transferred my nano reef fish into a larger tank , in the midst of emptying and catching fish, my yasha goby looked in major distress ... A bit like what your describing . From a closer look I was able to see that he had somehow inhaled large chunks on hair algae and strands were sticking out of it's gills. It took tweezers and lots of courage to dislodge 4 large chunks of hair algae down her throat and gills ... Took 2 days to recover...
I'm just saying ..could be anything... Not enough info to be of any help as is.