I think you're actually muddling some stuff up. Predators are notoriously boring fish. By their very nature they have to conserve energy: prey are not common, and catching them is difficult. Most predators spend a lot of time hungry.
The fish that are most active tend to be herbivorous species. These fish have lots to eat, but they need to feed constantly because plant material contains little energy. So they're always moving about. They often live in groups as well, because being sociable is a good thing if you're a herbivore (more sets of eyes looking for predators) whereas most predators are solitary (not enough food to go around). Sociable fish have hierarchical behaviours, so there's usually a lot of jostling about for status within the pack.
If you have a 60-litre tank, which isn't much space at all, your best bets for "fun" are going to be things like Tanganyikan shell dwellers. These are plankton-feeding cichlids that barely get more than a couple of centimetres in length. You can set up a colony of them in such a tank without any problems. The males will be squabbling all the time, or else flirting with females. It isn't all that difficult to breed them either, so you get reproductive behaviours as well. You do need to maintain excellent water quality and have an understanding of carbonate hardness issues when keeping Tanganyikans, but if you're a reasonably experienced fishkeeper, that shouldn't be too difficult.
Cheers, Neale
