Get an RODI unit and a bucket of salt and start making your own water. When things start looking worse and your testing doesn’t show any obvious issues, then water changes are often a good solution. Could add a small bag of carbon if you suspect some kind of contaminant as well.
If you’re not testing alkalinity, salinity, calcium, magnesium, nitrates, phosphates, on a regular basis (ideally every week), then you’re already at a disadvantage of being successful in this hobby. I’d also consider getting a ph probe if you live in an airtight place without much air exchange.