I'm going to give you my advise- this is what I did when I used to have that aquarium.
Those filters are not meant for Betta fish, or any long finned fishy friends. Crowntails will get their crowns ripped off by the intake, Halfmoons will get their fins shredded by the intake and the younger fellas can get pushed about by the outflow.
Not only that but the filters themselves aren't that amazing and very quickly grow algae on them due to the placement of the light on the hood. Not only that but the replacement pads are $9 for a pack of 3!
Here's whatcha do, buy a replacement filter! There is one called a Terta Whisper In-Tank Filter 3i .. You can buy this bad-boy at your local WalMart for a mere $10, and get replacement filter cartridges (pack of 6) for under $5. It's tiny too! It wont take up a bunch of room in your tank.
What's awesome about this filter is it's a bubble filter, it's intake is very gentle and it's outflow is also just as gentle. I put these in so many of the different tanks I had with all kinds of different tail-type Bettas and I never had any more ripped finnage.
This filter is more expensive at PetSmart but this is the first link I found, just so you can see what filter it is.