sick glofish

My pink glofish tetra has developed this weird bulb on their fin not too long ago, and from looking it up and testing my tank (PH, Ammonia, Nitrate, and Nitrite, which all came out normal/nonexistent — I’ve had this tank for a few years now), I assumed it was too much oxygen. I had been letting...

The issue: one of my glofish seems to be gasping, lathargic and slightly emaciated. Note: all my fish get fed regularily and I try to make sure all the fish get a decent share (without overfeeding them ofc). but the issue is that its just the one glofish thats acting like this. All of my other...