You can let it sit for a day without an air stone to allow chlorine to evaporate from your city water. Air stone will speed up process. If your city water has chloramines in it, then you’ll need an activated carbon filter to remove them.

Citric acid can be used to lower pH but as you’ve found out it’s not stable. Phosphoric acid (H3PO4) is a more useful acid found in most pH down products as it also becomes plant food as it breaks down.

Carbon in the air in the form of CO2 when mixed with water, creates carbonic acid. It’s a natural weak acid that helps dissolve limestone for example, releasing minerals. Using an air pump/stone in a high CO2 grow room environment will pump carbon in to your reservoir.