I'm kinda confused on what the question is... I'm not sure i follow what you mean by switching from tap to RO... if you just mean using RO instead of tap, you dont need to acclimate anything to RO...the animals wont notice any difference in the water.... The tap you're putting in now should be at least treated, so it wont have any chlorine or anything in it anyways... Only difference might be, say...phosphates, or TDS.
Or do you mean you originally started the tank on tap water and want it to be all RO now? If so... i stand by what i said. I wouldnt bother trying to get all that water out and putting new RO water in. If the tap water was gonna do any damage or put anything in the tank, its already there, so taking it all out at once isnt going to get rid of whats already in there (other than nitrates and the like). I'm not saying tap is good, but if it isnt killing everything as we speak, its not like you need to remove it all immediately. Just do your regular water changes with RO instead of tap and over time it will all be "RO".
That being said, if you're still overly concerned about it, there's not really much bacteria in the water column... Not sure why people think there is, but there's not. Theoretically you could change all the water at once and it wouldnt do any bit of harm... That is, assuming you've got all the params of the new water at least close to what they were from the old water so you dont throw everything into shock