I've only ever seen 1 Nano cube set up I'd even consider. My large LFS has this set up as a marine biotope with liverock and coral frags w/ 2 small clown fish and I've always considered this the best use of these expenssive space savers. Mind you @ 12 gal. this is no set up for Marine newbies.
If it were me and the info being givin' (all over the web) says nothing about Piranha and Nano cubes - I'd sell the Nano and use the money for a 55-75 gal. set up. I'm afraid that even with all the niffty gagets associated with Nano cubes the Nitrates produced by Piranha in 12 gallons of water would quickly build to unsafe and unmanagable levels. @ 1" a Piranha may be able to survive and remain heathy for a few short months - after that it will be an uphill battle with Ich and Cottonwool.