Regarding the gill curl, how can I confirm it’s not causing any distress. His behavior is normal and he’s feeding well, is there anything else I can do to confirm he’s alright?
You can do lots of things but the reason would state that the RTC is fine. Normal behavior is not a straightforward notion but if I understand you correctly, the fish is not showing any obviously abnormal activity, like gulping for air, breathing too heavy, swimming upside down or discombobulated, convulsing at times, upchucking feed, scratching, etc.
Normal appearance is also important, coloration, mucus layer, etc.
Appetite is the most telling thing. If it feeds well, chances are 99.9% the fish is alright.
Surely you can run other tests like a full blood test, EKG, upper endoscopy, and colonoscopy but again unless you are made of money and have nothing better to do and your local fish vet is just twiddling his/her thumbs, let's remain reasonable, no? (I too am just running a test here on your sense of humor
)
One easy test run on humans is the blood oxygen level, which is measured by that wired pincher thingy they place on your finger. This test would be relevant to your fish, as the risk here is that the fish might not be getting sufficient oxygen but IDK how easy it is to measure dissolved oxygen in fish blood... nor would I worry about it at all.
