I agree with tom, but i'll do it with more words
O2 and CO2 are independent (i.e., if you have 100% O2 that does not mean that you have 0% CO2. Your CO2 can still be 90%....) If you have 100% O2 and 100% co2 your fish will be able to inhale but wont be able to exhale. But this is a very rare thing to do.
Add yoru co2. Adjust your pH controller as necessary. Monitor your CO2 levels and have surface agitation to ensure gas exchange. Yea, gas exchange removes co2 from the water, but it also adds O2.