I'd second a seahorse tank.
IMHO the peacock mantis is too large for this sized tank. I had mine in a 30g (30" long) and it wasn't ideal, the 17g cube has even less length. They grow to about 6" and would need a lot of room to roam. A G. Smithii is a better choice as it's much smaller. The peacock is very prone to shell rot (it got mine, unfortunately
). You can't keep most corals as any decent coral-growing lighting will cause shell rot.