CO2 Diffusion for a 15 gallon - Improving Current Performance?

I have a pressurized system for my 15 gallon planted tank. Right now I'm running the CO2 line to a Hagen Elite Mini underwater filter (here), but I'm wondering if there is a better way to go about getting the CO2 to diffuse into the tank.

The issue is that, although the Hagen chops up the large bubbles coming from the CO2 line into much smaller bubbles, these smaller bubbles don't seem small enough. A large portion of them make it to the surface, to that point that there is a constant layer of bubbles on the surface of the water.

Are there any better options than what I'm doing now? I don't have a canister filter (main filter on the tank is an AC 5), unfortunately, so anything requiring the usage of a canister would most likely be pricier than I want. However, I wouldn't mind getting the Hagen out of my tank so I might consider getting a canister filter.

I don't know if this matters, but my drop checker is always a yellowish color, so I think the Hagen does an OK job. Just wondering if there is a better option

 

yes it is 4dkh. and it doesn't stay that color over night. that's the color it's at when the lights come on / go off (CO2 comes on ~ 2 hrs before lights come on)

 

well like I said I'm already using the mist method with a small underwater filter (I guess it's really just a powerhead but they marketed it as a filter). I just ordered a Rio 180 so that I can try Tom's needlewheel modification (the impeller on the current powerhead I have isn't conducive to making such modifications). But if just about anything will work then I'll just go with that.

If I could get the equipment out of the tank that'd be nice but like I said I don't have a canister filter so anything inline like that isn't really an option right now.