I have a tank with a 5 inch Green Terror that I've had (in the same 75g tank) for about a year. He was under an inch when I got him. In the tank with him are 2 1.5 inch male convicts, a ~1 inch Severum, 7 tiger barbs, and 4 clown loaches. I added 2 tiger barbs and the sev last week.
Normally he eats about 8-12 large Dainichi pellets per feeding, he ignores the 1mm smaller pellets I feed to the other fish. Yesterday he only ate two pellets and lost interest in the rest I attempted to feed. He wasn't spitting the food, but I noticed a larger than usual quantity of food coming out of his gills. Today he simply ignored all food I attempted to give him this morning, and did the same this evening when my girlfriend fed them.
I see no outward sign of trauma (nothing in the tank could really hurt him anyway, since he's 4-5x larger than anything else). My water quality is within normal parameters, and all other fish are healthy and behaving normally.
How would I go about diagnosing what is wrong? Should I start any sort of treatment?