"Your fish seems happy", Seems to be a synonym for healthy, used by pet lots of owners. 
However, many fish never actually do look "happy"...atleast to me. For example...
Arowana, Gourami, and Betta always seem to look as if they are discussed at something they just saw. (up turned nose, long unappreciative face)
Loaches and Tetras seem to look upset. (small oval mouth very close to eyes, looks shocked or upset)
Red Belly Pacu, Oscars (as well as a few other cichlids) almost always look tired. (mouth left open while resting, or even while just swimming)
Obviously this list is just what I see some might disagree, I've have generalized and left some fish out here are a few and why...
Different breeds of goldfish, some look extremally happy...some look mad (I have one with a fu man chu and I cant help but believe hes evil inside)
Plecostomus and other bottom feeders are in the same boat, but for a different reason. They look very happy when you see them attached to the glass in the middle of your tank, but look evil while hiding in the back corner trying to blend in with the rocks.
Angelfish seem to look aggressive, but you only really get a sense of it when you see them swim with there fast turns and jabs at the glass.
However, many fish never actually do look "happy"...atleast to me. For example...
Arowana, Gourami, and Betta always seem to look as if they are discussed at something they just saw. (up turned nose, long unappreciative face)
Loaches and Tetras seem to look upset. (small oval mouth very close to eyes, looks shocked or upset)
Red Belly Pacu, Oscars (as well as a few other cichlids) almost always look tired. (mouth left open while resting, or even while just swimming)
Obviously this list is just what I see some might disagree, I've have generalized and left some fish out here are a few and why...
Different breeds of goldfish, some look extremally happy...some look mad (I have one with a fu man chu and I cant help but believe hes evil inside)
Plecostomus and other bottom feeders are in the same boat, but for a different reason. They look very happy when you see them attached to the glass in the middle of your tank, but look evil while hiding in the back corner trying to blend in with the rocks.
Angelfish seem to look aggressive, but you only really get a sense of it when you see them swim with there fast turns and jabs at the glass.
