I've heard accounts of Cons spawning in 10g, however that's not really favorable. A 29 would work for a pair, and alot of people believe the same for a 20g long. I have two in a 55 by themselves, and they're doing amazing. As big as you can get is the key, don't stuff a fish in a container because it'll do "okay". Shoot for bigger so the fish can thrive, insted of just not dying.