Adding a few shells, IMO will lead to fighting over said shells. Especially with fifty fish and not enough to go around. To pair off fish, if you want to pass them on to others in established pairs, many people set them up in groups of six. I prefer to sell fish that pair off in groups rather than established pairs, because pair bonds aren't always as solid as we'd like. You could sell a pair to someone only to have them divorce and kill each other. Thats just my opinion of course. Having a place for them to breed and occupy would lend to the establishment of pairs once they were old enough. (To answer the actual question.)
Tim
