Ph Spike After Nitrogen Cycle

I set up my 80 litre blackwater aquarium last Sunday, today the nitrogen cycle reached the end, the ammonia and nitrite have spiked and fallen to 0 and the nitrate is up to 20 ppm at the moment. I noticed my ph spiked from 6.4 yesterday to 7.0/7.1 today, is this normal? There are plants in the aquarium that have been getting dosed with a small amount of co2 everyday, I was under the impression that co2 caused ph to lower though, not increase, is anyone able to explain how the ph has spiked like that? Also there's no fish in the the aquarium at the moment and I started the nitrogen cycle off with API Quick Start and also added API StressZyme+ to help increase the beneficial bacteria count in the aquarium. Thank you in advance for any advice offered with this 

If you are using liquid CO2 that is not actually CO2, it is glutaraldehyde and plants can convert that to CO2.