If you buy it from Walmart you still don't know what quality you get.
Using tap water and treating it can work for fish only --- don't try and keep corals but if you're using tap water with a saltwater tank you're really asking for problems.
RO and RO/DI filters out everything bad that causes nuissance algae to grow, can harm fish and easily kill corals --- heavy metals, phosphates, nitrates,etc.
Your water treatments treat mostly for chlorine which the RO process takes care of anyways if you buy the filter.
I always suggest airwaterice (just google it) as I've always been pleased with their units but theres a ton of good places out there.