Agreed. I've been a nano guy for almost 10 yrs. the key is weekly water changes. I never dosed either bc the weekly WC kept levels consistent. Use a good salt. I recently upgraded back to a 24" cube, 60g, but still smallish.i did run a skimmer in my nano because as solitude says, even 5 fish in a 30 create a strong bioload. I had a 28 and kept 7-8. My recommendation for a small tank is the aquamaxx hob-1. It's a hang on the back skimmer and skims it does. I didn't use it due to space restrictions. I went with the a Tunze 9002 nano skimmer. It's very low profile and fit in the back chamber of my nano with a little modding and it did the job. Those are my two recommendations.
As for your question regarding RO water, if you are referring to water for WC and top off, you absolutely should use RODI water.