I use straight tap in all my tanks. Charlotte's got some of the softest water I've ever seen. Generally I'd say that RO is a waste of time and money. Diana Walstad's tanks are at a GH of 17 and she grows the healthiest plants I've seen. All that extra Ca and Mg is great for the plants. If you're into soft water fish, then sure, mix a little RO with it. Otherwise there's not much of a need.