The problem is how do you define happy. Low cortisol? High oxytocin? Relationships between these? Low feeding competition? Lack of social or sexual conflict or the standing of a fish in a socio sexual setting (i.e. middle ranked are often less stressed than high or low ranked individuals in a social species). Certainly the error of fish keepers not understanding mechanisms/causes of stress and aggression leads many fish to be constantly stressed in captivity (be that the aggressor or the aggresse).
The other issue is many may assume that what they would like as a fish is the same as what is ideal for a non-stressful life as the fish. Anthropomorphism leads many of us to assume how a fish is feeling, even though the majority of reef keepers are not able to tell other than when the fish is showing extreme signs of distress. And that bring up another question. What does it mean to "feel" something?