You either need an RODI unit or you need to buy RODI water from somewhere, period. Well water is not in any way suitable for a saltwater aquarium. You have no idea what is in well water... minerals, heavy metals like mercury, arsenic, etc, and just plain old filth, not to mention what your pipes add to the water.
Public water treatment adds some bad stuff to the water, like Chlorine, but that's nothing compared to what they remove.
A system is only as good as the components that go into it, in this case, water, salt, additives, etc, and water is the baseline ingredient to that. If you don't start with a known standard and you end up having problems, they are going to be hell to find and fix.
Spend up, in this case you'll be glad you did.