Look in a store for a gallon of "Drinking Water" which is sold in the 69-79 cents a gallon or in boxes, three gallons for about $2. My limited survey of those containers reported upon yesterday indicated that one party wasn't telling all and did indeed add minerals. But the cheapest two added nothing andthey said exactly that in their contents label. Take a sample of that to your pet shop. If they have a TDS meter, all it will cost them is a few seconds to drop the meter in an inch of sample water and you will know whether that is demineralized or not.

For one or two tanks, probably one could by the RO water (which should be cheaper than water produced by distillation but in this crazy world, who knows?) I have several aquariums and during this lonnnnnng winter have plunked gallon on top of a tank corner or glass cover with just their 9-10 inches of hard airline tubing reaching out of the container and the attached 11-12 inches of the green silicone airline (very flexible) hanging in the direction of the tank. 20-30 gallons of that water has been put back into the tanks. If I was using bottled water that gets more expensive than having an RO unit (where the cost is someone hidden in the over all water bill).

Once or twice a year I have to change the fiber  prefilter and the carbon filter (which takes out the ammonia and chlorine and some other dissolved chemicals). It has been a while since I have purchased those things but the latest Foster and Smith catalog asking $16 for a carbon filter (which will be a lot less at the hardware store) and three of the sediment catchers for $18.49. The membrane on an RO unit for a unit like mine (rated 30 gallons per day, but usually producing less because of temperatures and lower water pressure) is in the $80-90 range. That is about the same as it was 10 years ago, the last time I changed it.

RO units have dropped about 50% in price over the last few decades  - sort of like what happens with an established electronic item.

That home production is small potatoes compared to the tens of thousands of gallons a day commercial plant produces, using pressure pumps and even finer membranes. There are also RO units, which can be leased for one's home. All maintenance is provided by the company, whose representative stops by periodically. That probably is as costly as buying gallons at the grocery store though one could run a lot more water with the RO unit.

Maybe take your tap water, aquarium water and if they don't mind a couple commercial bottled water samples (one cheap, one the more expensive name brand) along. All can still be tested within a couple of minutes.

One of the uses of a TDS meter I had not anticipated is monitoring how TDS goes up on an aquarium which hasn't had recent water change. It may go up even more if a person is just using tap water to replace evaporation. You may recall that in that "Mystery Death" article in Immediate Help, a guy had TRIPLED his tanks hardness (TDS was probably even higher!) to a lethal level in just a few winter months.

A positive use of the meter: I have adjusted some quite different tanks (gradually adding RO water for the high ones and tap water or tap water and tap water and some cichlid salts for low ones). Then I could shuttle fish from one tank to another like crazy. They were always going into a tank with the temperature very close to theirs and a TDS 5-10% higher than they were used to. Just the changing of water. while doing that, clearly improved the vitality of one tank's fishes.