250 gallons seems like serious overkill for one betta...why not divide it a few times and have several very happy bettas in there? maybe vary the sizes so you can have a few normal sized bettas on the sides and a giant or king in the middle?
also, how deep is the tank? i remember my 300 gallon iguana tank was nearly three feet deep, and i can imagine that'd be quite a struggle for bettas to get to the top to breathe, unless the tank was only half filled. (that might be nice though, to have some semi-aquatic plants in with the full aqua plants and give them room to grow above the surface.)