Well, firstly, I don't think fish conciously realise that they are in a small tank; due to internal reflections it looks infinite and thus like the sea. However, I do realise that being in cramped conditions could be bad for them, but I don't think the fish themselves realise it.
I also think that although fish don't have a short memory span, this doesn't mean they have feelings or are "clever" in the human sense. I'd imagine that they would run purely on instincts, as most animals appear to do so. The only animals that don't are ones with brain sizes approaching that of a human (eg. elephant, dolphin). This would mean that they are unable to, as such, have feelings such as boredom. Finally I think it is unlikely that they would feel fear; no animals seem able to do so, due to their inability to anticipate.