You can always supplement your DIY CO2 injection with daily doses of Flourish Excel.
I run 3 DIY CO2 devices on my measly 29 gallon tank, and still add Excel daily.
I have never been able to get my CO2 above 17ppm.
Mind you, I have little to no water surface disturbance, so I know the CO2 is being dissovled in the water.
Just my 2 cents...