How do you define happy, right? I think sometimes people think, alive = happy, dead d/n = happy. I have two ocellaris in my ten, hosting in frogspawn, who appear to be happy-although they never tell me so, the little ingrates
But they have warm water, daily food, and enough hiding places in my 10g to at least to keep them entertained. I think, THE BEST we can do as fish keepers, is to keep them safe, fed, and warm. It's up to them to decide if they're happ or not.