Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Yes. The nitrifying bacteria use carbonates as their source of carbon. Most organic processes such as decomposition, or composting (fallen leaves, fish poop, left over food...) will tend to have an acidic reaction.
I would test each batch of water as you are getting ready for a water change and add enough baking soda or potassium bicarbonate to make sure the KH is at least 3 German degrees of hardness. You might not have to add anything most of the time, but the tap water is not always the same.
Test the tank every few days through the week (or however long between water changes) to monitor what the KH is doing. You can add baking soda or potassium bicarbonate any time to boost the KH a degree or two.
If the water company is adding something to keep the pH up (sounds like it with low KH but high pH out of the tap) perhaps something in the tank is removing that material from the water, then the low KH allows the pH to drop.