Room at the Table

One of the biggest tells I use for general fish health is more than just whether or not a fish eats. It's so much more than that. 

Take the Copper Banded Butterflyfish for example. The first week or two you have it, it's probably not going to eat. That doesn't mean that you should worry about it yet. 

In my tanks, I like to use feeding time as a roll call for my fish. The reef tank is really where this comes around. Most of the fish gather around the frozen cube of food I have held in my tongs, though some will never join the dogpile. Some hang back waiting for bits to fall off the cube. Some don't want to stray far from their hides. It's a general deviation from normal behavior that gets me worried.

My exquisite fairy wrasse is a good example. He settled in quick and is very food forward. He's almost always the first one at the cube. He'll often feel like he smashed into it with how quick he hits it. If he was hanging back for any reason, I would immediately notice something was off. He could still be eating for me to start to look for any changes in the tank. 

Other fish have a different vibe. My filefish sometimes likes to get in on the chaos and sometimes doesn't. I don't worry if I don't see him in the mob, but I do worry if I don't see him near. 

While not loud about it, fish can be very clear about something being wrong long before a problem becomes irreversible. Food time is your best baseline for that.