If the animals are already doing well in tap, wouldn't it be easier to just leave them on tap? Once they've acclimated, that is. Normally when we use RO water, the transition is done because we want to change at least one parameter compared to our tap water, and usually, it's more than one. I switched to RO for some of my tanks because my tap water is too hard and alkaline with too high of pH for some of my fish. If you're going to be doing the same, I'd recommend doing a series of small water changes with straight RO until the desired parameters are achieved. Then you can just maintain by premixing your RO.