I have kept sunfish and know of other people who keep sunfish as pets. I would not use anything smaller than a 28 gal. The larger the better especially if you plan on keeping more than one, even for little ones because they grow quite large. In my tank I I have several rocks piled into 2 little caves where they like to hide/sleep. They eat smaller sinking pellets (it takes them a couple of days to get used to it and you can feed them small minnows or worms or insects that you find as a treat). If you plan on giving them any friends, (Algae eaters etc.) you should make sure they are too big for the fish to eat because they will try to eat anything that fits in their mouth. here