What kind of filter is it? If it's rated for a 30g, it should be fine, however, I've never really had good luck with a HOB with my Bettas. It seemed to cause too much current, but they've always been rated for almost double the tank size. I think those small internal filters work well with bettas as long as the current isn't too strong. Since they like more calm waters, those internal filters don't seem to cause too much current.