Have a 150g with a CO2 regulator hooked up to the return side of canister filter running through CO2 reactor. Have it set to ~ 2bps. The drop checker does not change from blue even when placed closer to the filter return (where CO2 comes in to tank).
Overall, water is high PH (well water through water softener PH of ~ 8.5). the CO2 does not seem to be changing this at all.