Filtration for a 2.5g?

[Bfunk89]

I have my bettas in a divided 2.5g tank but I'm having troubles with keeping the water moving, but not too much current for the little guys. I tried a small air pump meant for up to 5g (smallest I've seen so far) with divided hose to two airstones (one for each side as in the pic) but its causing way too much bubbles and movement for the bettas. I tried just one stone but then one side was crazy bubbles and the other almost still. I've come across a couple small filters but I wasn't sure if the divider would negate the filter on the other side of the tank.

Any suggestions for keeping a gentle current in a 2.5g tank?

[gklaw]

pm Mykiss. He carried mini internal filter for $10.