IMHO four gallons will not change your calcium level. You might want to consider doing atleast ten percent then test in a few days. I have never seen that much of a drop in Ph from GFO either. How much GFO are you running. I am assuming that it would take a large amount to effect the buffering capability of your water column. It is important to note that your water will become acidic once it comes close to 7.o. 7.9 is a little low but not horrible. What is your alk at ? Alk and Ph work together but to much is also not a good thing too. Good luck