Silver Dollars

Yep. And same goes for freshy tanks. I used to have a 55 that I kept native species in (trout, bass, blue-gills, channel cats). Would take them back to the lake and release them when they got too big
But as said, they will get big and need a bigger tank *eventually*. A lot of people get weird about that, but I see no problem with it. Heck, I have my Hippo in a 40 and its doing fine for now But if you have a good LFS, they will usually trade you straight across for the smaller ones, or even give you a little credit.