RO water is just water, no minerals. Most tap water contains calcium, which is an important nutrient for plants. You can use RO or distilled, but you'll have to make up all the missing elements -- all the major AND trace elements.
I use tap water. A little note here: due to calcium content, my tap water has an EC of 1.2 right out of the hose.
Pretty much everybody uses regular RubberMaid tubs to hold nutrients. Wal Mart nearly gives them away!