You could try using a bell, ladder, or other method of getting the CO2 into the tank, rather than feeding it directly into the filter.
The only way to lessen DIY CO2 production TMK is by using less sugar, and less yeast. You could try getting another bottle and pouring half the solution in, diluting it with water, and seeing how that goes, but I don't think it will solve the bubble problem. I use a small piece of bubble wall to diffuse the CO2 in all my tanks (I know it's not the best way, and I'm careful to clean them every week). Some of the CO2 in the 65g gets sucked into the FilStar, and it periodically releases a cloud of tiny bubbles - I tried feeding it directly into the filter, it didn't work well at all (many bubbles, and very noisy).