If your pH is only changing from 6.6 - 7.1 it doesn't sound like you have enough co2. I could be wrong but I'm pretty sure that with a change that small there's no way you could be injecting enough co2. But if you have low light it won't matter all that much.

Since I'm running diy, the only way for me to keep co2 levels high enough is by letting it run all night, otherwise after 12 hours of dropping it takes another 12 hours of injecting to get it up high enough again. My fish don't seem the least bit stressed, I do have quite a bit of circulation so I guess O2 levels are high enough.

I've had lots of issues keeping co2 stable in the past, with pH fluctuations of 1.0 - 1.4 in one day. Fish didn't seem to care at all.