You can use RO
Reverse osmosis / deionization water available usually at a Saltwater fish store for around 50 cents a gallon.
Or use distilled water mixed with your well water - you might try 1/3 your well water and 2/3 distilled and test it. you don't have to use it in gallons amounts to test, just like a cup size 2 distilled cups one tap/well water of each.
Also I know most people are super happy about having well water for their aquariums and don't bother to use a dechlorinator, but some dechlorinators do more, like bind heavy metals which could be bad for your fish, but especially bad for invertebrates.
Some people have had their well water detail tested and know what is or isn't in the water, in which case you might be just fine.
I recommend using something like Prime water conditioner to be on the safe side if you aren't sure!