Choice one: 'Re-make' the water after the R.O. process to add back what is needed. This is a really GOOD option, as you will know exactly what is in your water. The down side is, it's expensive to do it this way, compared to choice two.

Choice two: 'Season' tap water (I.E. Let it stand a couple of days in a bucket) and use that, instead. This will allow most of the nastyness to evaporate out of the water before you add it to the fishes environment. Then, use that.

Stratigy: Use the remade R.O. water, or the seasoned water to fill tanks, and to replace water lost to cleaning via vacuum. Use straight R.O. water to refill water lost to evaporation. Why? Because of the mineral content. If you add water back in after evaporation, and that water is seasoned or remade, you will in effect be adding more minerals to the tank than what is 'correct' for the amount of water. In otherwords, the ratio will be off. This ultimately, over a long period of time, will make the fishes environment unlivable, and leave you trying to figure out why all the fish died.