If you feel like spending the $$ on purified water and calmag, IMHO that is a better bet than tap water that you distrust. Letting tap water sit doesn't really change its chemical makeup significantly. But the burn symptoms were definitely an overdose of Magnesium and while you were sort of on the right track, it is as you can now see much safer to treat Mg def with a foliar spray, and keep your reservoir free of stuff that you don't absolutely NEED for plant growth, especially when you're on the uphill side of the learning curve!

RO + CalMag Plus + a quality hydro fert + plenty of oxygenation to the root zone + good temperature and pH control and ample light = your basic plant needs.