I don't think it really matters if you diffuse it with a powerhead, glass diffuser, or through your HOB filter. The filter will still be gassing off some of the CO2 as the water exits the filter regardless of the diffusion method. If you're maintaining at least 18-20ppm with DIY CO2 and HOB filtration, then you're doing fine, IMO. The plants should be able to benefit from it more so than no CO2 at all.
