Planted Tank Obsessed
Join Date: Dec 2010
Location: Binbrook, Ontario, Canada
CO2 will cause your ph to drop fairly quickly and then rise again when it goes off - this is normal and does not affect your fish. Mine fluctuates as much as 1.5 down and up.
On the other hand, drastic fluctuations in PH caused by changes in water chemistry can have negative affects on your fish, for example when you do a large water change or add substrate or certain types of rocks or chemicals to your tank.