For a male betta, it really depends. I've seen males live with many other fish in a five gallon tank. I've also seen males that would fight to the death with anything in a 20 gallon tank. I really depends on the fish and how teritorial he is. My suggestion would be to get 5 gallon tank with an under gravel filter and put a clear divider in it if you want to add other male to the other side. You can add an opaque divider plate so they won't see each other, then when you want them to fight, pull the opaque divider out and let them fight through the clear one. They can't physically touch one another, but it's fun to watch. Be careful with this though as it can stress the fish out big time. Total cost for this set up is about $25. You can do this with a 10 as well, but it is going to be more expensive.($50) Go to Wal-mart and take a look at tanks. They have some pretty good deals on them. If you do put him in a 10, add a cheap fish like a tetra or white cloud and see how he reacts. If he kills them, you'll know and won't be out that much on fish.