Hi and welcome to land of co2..
Your ph will drop as co2 forms carbonic acid which lowers you co2.
It is very important that you water has good buffering capacity as in KH (carbonite hardness). About 70 ppm or 4degrees hardness is an average number needed. You will need to test your water with a kh test kit. If it is too low you can add baking soda (NOT baking powder) 1 tsp 10 gals very slowly over a period of time until you get a good kh reading. Once your kh has risen above 70ppm you won't have too worry about your ph crashing. I just treat my water during water changes and then pump in the new water already kh boosted.
Best of luck
