Just something I would like to point out... I'm pretty new at the whole betta thing, but I have two bettas in a 6 gallon with a divider. I am noticing that it looks like the water on the side of the tank without the filter looks dirty on the top, while the other side looks clean.
Not sure why this is... but I'm sure the answer is either get two smaller filters or get a different divider.
Currently, my divider is this see-through thing I found in a pet store. It actually looks pretty good, but I am wondering if the holes are just too small to allow filtration on both sides.