Is there a way to de-scale stainless steel that has had regular (not distilled) water?

Try some white vinegar mixed with water inside the tank.

I have very hard water and lots of calcium deposits in my tea kettle; an occasional white vinegar bath cleans it out.

I would agree with @gailcalled. The deposits are probably calcium carbonate, so something acidic like vinegar should clean it out.

I third vinegar.

I use it full strength to rid my home of hard water stains & lime scale buid up. In the coffee pot, the tea kettle, the stainless steel sink…I also use it to wash my dishwasher & washing machine.

Your tank should be clean in a matter of 15–20 mins. Depending on how much lime scale you have, it may take a second treatment.

Every few months I fill a mug with white vinegar and hold it up to my shower heads for a few minutes in order to dissolve the gunk that collects and clogs the holes.

Thanks all, vinegar it shall be.

I used hydrochloric acid on a stainless pot used to boil water, cleaned it out good. You can get it in lowes or home depot as pool acid.