I know I'm coming in late, but I've got the same problem. I inherited one neon and got 6 more smaller one's. The bigger one hides in a log and chases other fish out. He always kept to himself, but lately he and all my other 6 have been chasing each other and fin nipping. Water parameters are good. Temp at 77, pH a little low at 6.0. My tank is about one month old. It happened suddenly. .