People used to think that animals were incapable of any emotions... that dolphins, elephants, apes and parrots merely copy-cated and didn't have any imagination or thought of their own... that pigs were stupid animals only good for their pork... that fish didn't feel pain.... all which have been disproven.
We're learning more about our world and the creatures in it everyday.
Every species has a sense of 'play'. Lions, wolves, elephants, mice, squirrels, birds.... whether this play is to practice survival techniques or simply to have something interesting and stimulating to do, many creatures have a sense of frolick. Is it such a far stretch to say that some species of fish may have similar traits? We know that fish can recognize humans as food-givers, even to the point of knowing one person from another or knowing family members from strangers. Some fish can even exhibit a level of 'trust' and allow themselves to be handled by some people and not by others.
Until more research is done on the matter to disprove it, I think fish... at least some species of fish... can exhibit play and enjoy toys in their tank. Things to tug on, flash against, chase and bob around the tank 'just because'. And I don't think it's silly to try creating fish toys just to see if this theory would hold water.