Me too. Wells tend to fluctuate more than city water due to city water treatment facilities. The minerals in it can and do change concentrations. Sometimes whole filters last for months, others I change the stage 1 (sediment) filter every weekend.
You bet your bippy it can. The R.O. system I use is made for gardeners, its last stage adds calcium and magnesium to about, 30 PPM when new and settling done to about 15 PPM after some use. This is it if you are interested. I bought it at Home Depot.
Before September '17 I hauled three 5 gal bottles per week from several different stores. That water was totally empty water. It wasn’t hard to adjust the pH as long as it had nutes mixed it. Before mixing, it was difficult to get a stable pH measurement. It usually ended up to read close to 7.0. I could put just a drop of PH Down and the pH would drop to under 4.0. The first time I made pH water, I dumped out all three bottles of water and went to a different store to refill. When it happened with the replacement, I figured out what was happening, but I still chased the reading up and down until there was enough in the water to allow a stable reading. This is the reason I bought the one that added cal/mag. The pH water measures just fine now.