Hi!
I am going through this EXACT issue right now. I was thinking of making a new post of my own, but maybe my experience will help with your guy too.
I bought 2 GTs maybe 4-5 months ago. Got both of them eating NLS large fish Thera-A pellets within a few days. Great. 1 Terror is mine, 1 is my boyfriends. First week or so of quarantine they split a divided 40 breeder; I setup another 40 breeder and moved my GT over to his own tank. They are both solo fish, tanks filtered with Aquaclears, cycled, pots to hide in.
As soon as I move my GT, he turns down NLS consistently. Spits it, doesn't give it a second thought. I tried soaking in bloodworm juice, stuffing into krill or tilapia, different pellet sizes, Seachem Entice, turns it all down, total refusal. I fasted him for nearly 2 weeks - he still consistently rejected NLS. I eventually fed him a gauntlet of crappy cichlid tetra sticks, tilapia, krill, and bloodworms. Eventually he began rejecting these foods also. Meanwhile, the other GT continues to eat NLS and has outpaced mine in size and condition. Mine gets that slightly hollow belly look that I see in your photos also.
I treated with Levamisole and Metronidazole (heavy doses) for about 10-12 days. I am still not sure if he was ill, potentially with an intestinal protozoan or other parasite. I am not even sure if my treatment was effective!
I bought a bag of Hikari sinking carnivore pellets, a food I've had great acceptance of in the past. I started soaking these in a few drops of water+Vita-chem (not a ton of water, just enough to saturate pellets). He started eating these post-treatment. I literally spoon-feed the pellets one by one, and I don't feed them too fast. He eats one, I walk away for a minute, I come back, I feed another. Repeat until I see the slightest drop-off in appetite. I recently started soaking a few NLS pellets along with the Hikari+Vitachem. He eats these now too! I think we are maybe on the road to recovery.
