I hesitate to say I can tell when the fish are "Happy", however I know for a fact when they're pissed. With luck they spend less time mad and angry, than not indicating much of anything.. and I'll call them "Happy" fish. There was one time I knew for sure my Tang was happy. He was in his corner of the tank, resting on the sandbed, and had labored breathing, short, shallow breaths. He had just finished off about 3-days worth of Dried Nori from BSD (and it was a fresh batch from BSD, still moist even), his belly was so fat, I'm surprised he could even swim. I knew emphatically, that he was "Happy" for that moment.
- Mac