There is a yellow tang in this 55G and it is behaving normally. Other critters are large snails, BTAs and urchins, all ok.
Being a fish only tank with no corals, I don't pay attention to water parameters as closely, I should check them tonight.
This Lion gets enough to eat, it knows to stop when it is full -> usually 1 or 2 small pieces ( small enough for it to inhale it whole) of chopped up shrimp would do that. It also gets frozen krill and mysis. Fed no more than once in 2 days.