RO is not necessary, BUT, it removes the chlorine ... chlorine won't harm your plants, but it does kill the beneficial bacteria ... so, if using chlorinated tapwater, don't waste your money on microbial additives (Big Bloom, Tarantula, etc. etc.), they'll be killed as soon as they hit the water ... leave the water uncovered for a couple days, so the chlorine will have time to evaporate, then you can use additives, if you choose ... :smokin: