I cant get my co2 at level I want. I have a sump as a filter, it is covered. Is there any tricks for setting this right, or should I get a canister filter?

I'm injecting co2 using a DIY pvc reactor. The sump is a 20g tank, flow is about 400gph. The tank is 120g.
I have to do like 15bps of co2 and it's still very low.