Long Spines are neat, but they require a pretty high water quality and they get big enough so that in time they could take up the majority of that tank. In any tank under 40 gallons or so, they will need supplemental food. Also, a couple banggai cardinals would be much better tank mates for a longspine than clowns, as they will actually hide in it's spines. Its actually a tank I've always wanted to try, was a couple longspines and a school of cardinals. Neato.