Hello ! I would prefer tap water rather than RO water except if you'r planing to add cal-mag with each watering. RO water have an ec/ppm near to 0 ( it is suposed to be 0) but tap water is generally around 0.3 whith a good amont of cal-mag : the choice is up to you. Foxfarm Ocean forest is a good choice , no need to feed during the first 2 or 3 weeks as the soil is already pre-fertilized after. Your RO water is perfect to mix for a foliar spay. Hope it will help you 😁👍