But what evidence is there that a fish is actually cognizant of such conditions? If reactions do not extend beyond the instinctive level and the organism has no rational concept of what is happening, how can the term "happiness" even be applied? It is after all a largely human construct, based on the way that the human mind works. Can such humanization's be applied neutrally to the capacity of organisms with brains which are some orders of magnitude lesser developed?
On such grounds one could just as easily make the argument that keeping wild caught fish at all is a terrible choice, because only the largest of public aquaria would be able to house them in a manner which sufficiently emulates their wild habitat - even then the constraints of a closed system play some role in the development of the internal ecosystem. But then one could bring up the argument that perhaps in some instances, aquaria serve a better home for an organism than where it is found in the wild... pollution, climate change, invasive species, etc.. But then where do we draw the line? Under what mechanisms are we to determine concretely that one is better than the other, and if one is appropriate at all?
While it is easy to imagine your disdain for the human condition and it is certainly shared by many individuals on this website, the ability to logically come to solid conclusions by scientific exploration (however tedious and unapproachable it may seem to some), should not be readily dismissed simply under the guise of "ethical husbandry".
Some might be more inclined to side with the "unspoken for" creatures, while I personally would err on the side of caution and treat them than no more as they are.
It seems easy enough to say that "ethics and morals are only a matter of opinion", but if anyone would like to open that argument to the floor I would be more than happy to reduce it to a pile of unsubstantiated drivel. The human mind is a beautiful thing