All of these treatments were over 5 day period? This is why I would never use a brand that doesn't tell you what is in it. If it is copper sulfate and after reaction complexes with ammonia toxicity increases. Prime is likely making ammonium sulfate freeing up the copper part. Ammonia in that case could possibly be read by some test kits and still be safe. Reading other places says its chelated copper. There are many ways to "chelate" copper. Can't say anything definitive without knowing actual ingredients. If there is an excess of ammonia it will make some complexes. Also the kanamycin has 4 amine functional groups that could be kicked off in a reaction. Adding to the ammonia in the long run as well as just being in a bare bottom qt. One other thing metro if he ate any while you were not looking possible can cause reduced appetite as well as stress can. Copper, the metro, the ammonia, and the stress can all lead to them not eating. Especially with tangs. Some times just being bit by another fish can cause a fish to flash. Also if the salt isn't mixed enough I've seen flashing from that. He doesn't look skinny to me. Some things to think of are aggression, constipation, other issues. I did look at your other post. Red spot on fin looks gone now. To me this just seems like too much treatment in this time frame and handling of fish. You don't know what is going on but you treat with an arsenal of things? Sorry if that seems coarse. Sometimes doing nothing will be good for a few days. Wait a week and start over with copper and a different choice of med if any red comes back is what I would do. Biggest thing now monitor ammonia and keep up with that in qt in the mean time.