I certainly would start checking the GH and KH.
Here is why:
Water that passes through the ground dissolves whatever rock is there. If the rock is high in limestone or related materials then the rock dissolves quite easily and these minerals show up in the water.
Calcium carbonate and magnesium carbonate are the most common minerals in limestone and related materials.
Calcium and magnesium are measured by the GH test.
Carbonates and bicarbonates are measured by the KH test.
Any other minerals would show up using the TDS meter. This is Total Dissolved Solids.
Carbonates are the most common buffer of pH in the aquarium. When the KH is high then the pH tends to be high, and does not change easily. If the KH changes, then the pH can change. I think this is what is going on in your tank.
Caltrate is calcium carbonate. So you are adding calcium and carbonates to the tank with every pill. The GH test would show if the Ca is rising. The KH test would show that the KH is rising. Rising KH = rising pH (almost always).
Hard water (high in GH or KH or both) is good for snails, up to a point. You can go to the apple snail web site and get the optimum levels for them.
Blending your well water with distilled (or reverse osmosis) water is a good way to keep the GH, KH, pH and TDS exactly where the snails like it, even when the well water may vary. This means you will need to test every time.
As you add fertilizer or anything else (caltrate, snail food) to the aquarium the TDS is rising. Doing water changes is the only way to drop this. Some people use the TDS as a way of knowing when to do water changes. Allow the TDS to build up slowly then do a water change.
Top off the tank with distilled or RO water so you are not adding more minerals to the tank.