If you are only doing 10% partials once a month, then yes your pH is going to drop via natural processes in the tanks. To keep your pH up and your water as close to your source as possible, you should do larger water changes weekly.
Your pH drops because the KH, which keeps your pH stable, is being used up. Your biofilter needs the KH to survive. Without it, it will die off. The drop in pH is your signal that you really need to increase the number of water changes you do.
Ok this morning I did a partial water change and added baking soda to both my tanks. This evening I retested the ph in my tanks and the results are much better.
The 10 gallon is between 6.6 and 6.8. The 30 gallon is between 7.0 to 7.2.
Now all I have to do is get my nitrates down. Will that do down after I do another water change? I plan on doing another partial water change and adding more baking soda in a few days. Probably Thursday.
I heard that adding baking soda to the tank will bring up the Ph and it's worked so far. I did buy a ph up product but I think I'll stick with the baking soda for now. I plan on getting new filter cartridges tomorrow. I forgot to get them tonight when I was at the store.
What Is your Tap pH after it has sat out overnight ????????
The baking soda can be effective to add Kh but probably isn't necessary. 10% changes monthly would explain every problem you have mentioned here. Do several small volumewater changes (10-20%) on a daily basis until your nitrates are down and your pH matches tap. Then figure out a routine to maintain that condition. for me it's 50% or more weekly (I feed heavily to promote snail production).
The tap water test will tell us if you have enoughKH (Roughly). 6.8 is still extremely low and the tank stands a good chance of rebounding back to the previous situation.