When I had my 2 tanks like this, I pitched the UGFs right away. It was way too strong for them unless I somehow pinched off the airhoses - and then it was a teeny little bubbledrip. THEN.. (as someone said here already) I was terrified that with that little hole left in there, they would get curious and get stuck down there. So those things went in the garbage right away.
Really, with a betta when you're doing 100% water changes, these things serve NO purpose whatsoever except for surface agitation, and that's only to combat a slime buildup that gets up there - it's not anything sinister. The slime coat at the top of the tank happens on ALL my betta tanks regardless of the size, because with the exception of my little Jasper, nobody has a bubble stone at all - and no filters, either except in the 10G tanks.... Jasper's had a bubble bar since day one - it's a 4" bubble bar that has only enough air flow going through it to make one teeny tiny little bubble stream. He LOVES to hover over it and if I take it out he sulks.
So - if your betta doesnt' seem to mind the current it makes, they're fine. Just beware if you take part of it out, take it ALL out or you'll be leaving a little death trap in there for them.