Hi,

Yes, the ph will fluctuate.

This is a nomal scenario in nature and in c02 enriched tanks as the 'sun' gets higher, plants wake up, optimal c02 levels are reached, etc. PH changes more than people think IMO.

Here is a thread that spoke to ph shifts and fish health.

pH shifts and fish health due to CO2


I go from 7.3 at night to 6.6 during the day using c02 with no adverse affect on fish, inverts, plants.

Just have your extra powerhead turn off and the c02 come on about 60-90 min before lights come on to get the c02 back in the tank again, as the level will be low after all night of 02 enrichment.

Hope this helps.