Originally Posted by Hilde
Easiest fish to keep in a small tank. Just wouldn't put any other fish with it. They don't need filtration.
This doesn't mean they don't have a bioload, and don't suffer from ammonia burn and toxic conditions the same way other fish do.
Betta are really neat fish, sure, but they still deserve healthy living conditions, and will always do better in a filtered tank. It's more the aeration they can do with out, as they rely heavily on their Labyrinth organ.
But possibly the point was that they do very well as solitary, focal point fish, and as a solitary fish in a well planted tank, their bioload is easily countered by the plants. I've seen some very lovely nano tanks with Betta as the star :]
 And hey look, here's a fellow forum member with a lovely set of HMPK Betta for sale!