Twelve gallon tank...I would say channa gachua...all by it's lonesome....get's to about 4 inches...in a bigger tank, i.e., 40 gallon, it can grow to 6-8 inches...Very hardy too, air breather so you don't need filtration, low maintenance, can survive in a stagnant water, but likes also running streams...so take your pick and splurge. Undergravel should be enough..and will eat pellets.