Oh. believe me, I know about smaller tanks and water quality. For this tank, I don't necessarily want tankmates. I've got three other tanks, so I'm not looking for quantity. I just want two bettas. I was thinking I might take a ten gallon, and like you said, use krylon to paint a divider. However, I was thinking about having a clear divider siliconed to the sides, and something else to hold the dark one up. That way, I could let them see each other whenever I wanted. And, of course, I would drill holes in the dividers to allow for flow from side to side. An easy way to do this is have the filter intake on one side, and the water spilling into the other side. That way, the water has to flow between the two sides.