I try very hard to avoid using words like always or never on this or any forum, but I do have a lot of experience with clowns and would say that they almost always thrive in pairs. I don't know about emotions when it comes to clowns, but I can tell you with very little fear of contradiction, that most clowns will be stressed (if not unhappy) in a tank either by themselves, or with more than one other clown. I breed clowns so I have had the opportunity to observe thousands of them in conditions ranging from 300 3/4" juvis in a 30g grow out tank to a single sexually mature female clown in a tank by herself awaiting the addition of a suitable mate. I actually believe that sub adult clowns crowded into a smallish tank are happier than any clown that is isolated. Even with all the bickering in a heavily stocked grow out system, most of the juvenile clowns will grow healthy and strong, and the fighting at that age is more akin to posturing than the all out wars that inevitably will occur when they reach sexual maturity. But more to answer your question, all my new clowns that come in from outside sources are quarantined for 6 weeks prior to introduction into any of my systems. Even during this relatively short period of isolation, their behavior is dramatically different than when they are in with another clown. They tend to be more skittish, and generally just look more stressed, than paired clowns. Clowns that I have had isolated for longer periods of time say 6 plus months, seem to loose all their character and spunk. I had one that ended up swimming upside down in the top corner of a 55g and wouldn't even turn right side up to feed. I literally think that isolation drives clowns crazy. All that being said, singles do seem to fair OK in a system with other species and and inverts. It may not be the most desirable interaction for them, but it seems to keep them occupied enough.
All that being said, buying a "pair" of clowns is actually a bit more difficult than many hobbyist think. Generally speaking, If you buy 2 1.25" clowns at the same time from the same LFS, they will take much longer to actually form a bonded pair than if you buy two clowns with a significant size difference. You also run a higher risk of destructive fighting when they both reach sexual maturity around the same time. The battles will tend to be more aggressive, and the risk of serious injury is a real threat. Clowns with large differences in size will reach sexual maturity at different times, and the larger of the two will become the female and the smaller will stay male. The struggle for dominance will generally be much less pronounced as the smaller clown will submit much more readily. That's not to say they won't ever fight, but it is far less likely to go beyond chasing and posturing.
Sorry for the long winded response, I just love this family of fish and find it hard to get to the point.
Short answer......yes I think they are happier in pairs.
-M
