Last Monday, I had a friend give me 3 of his fishes that weren't compatible with his fish. I moved them in mine and the columbian tetra wasn't compatible with mine so I went to the LFS and gave it to them since it wasn't going to work in mine. One of the glo fish Tetras he gave hasn't been eating. I've tried everything. I have Tetra Flakes (primary food), aqueon tropical flakes, hikari micro pellets, hikari bloodworms, hikari brine shrimp with spirulina, hikari sinking wafers and pleco wafers from my failed attempt at keeping an oto. So seven different foods and nothing. He swims and acts normal. Always active, swims back and forth normally like my other glo fish Tetras. Parameters are at 0,0,20. Water changed twice in the last 7 days with RO water with replenish, prime and pristine in the last week and still no help. Double dosed stress guard to help from the 5 minute move but nothing. Worried it's going to die if it doesn't eat. Still acts normal which is weird. The ONLY issue is he has a black fin. Just one of all of them. From looking online it looks like it could be be ammonia or nitrite poisoning. My tank is very cycled since January and I do water changes every 5-7 days at minimum. My friends tank, while I haven't made sure but he's had it for over a year so I would think it's cycled. It's not getting fat or bloated so I'm thinking parasites are out of the question. I dosed the tank with prazipro as preventative in the beginning to make sure the fish dont introduce anything to my tank. But even then majority of it has been removed through water changes. No carbon in my tank. I'm at a loss. Please advise.