I am runnung 80% RO/DI and 20% tap but it will depend on your tap water. I am on a well so no real problems other than my water is so hard I think it could break glass LOL. I am useing tap to help stabalize the PH and to add some minerals back in.
Water chemistry varies. In mid Michigan, Equilibrium is the usual product suggested for dosing RO water. Your goal will depend on what type of tetras you are planning to breed. You'll probably want a holding or water aging tank as rain is the usual stimulus for tetra spawning. A 50% water change substitutes for that in tank rearing.
Talk to knowledgeable staff at a local fish shop.
Find a fish or planted tank club or group in your area.
Salt Lake City is large enough to have a couple.