Ah, I see that you already have the air stone, and an air regulator. So instead of having it go from:
air pump->air regulator->air stone in sponge filter
I would instead buy a T fitting, and connect it like so:
air pump->t-fitting->air stone in sponge filter.
And then, on the second part of the t-fitting, I'd add the air regulator. This helps you better fine tune the air going into the tank. Air will preferentially go through the path of least resistance, so in this case, it would flow out from the t-fitting into the surrounding air. BUT when you add an air regulator, you can fine tune how much resistance is there, and can manage how much air goes through the sponge filter.
Alternatively, how big is your air pump? It's rarely an issue of the sponge filter, but usually an issue with the air pump, which manages how much air gets pumped into the sponge filter in the first place.
So many fish/plants/inverts to keep, not enough aquaria.