Another downside of using straight rodi water is that it is very unstable with regards to ph. The minerals in tap water help to keep the aquarium ph stable and at a safe level. If you go straight back to all tap 2 things will happen. 1 your ph will change rapidly and most likely bounce around before settling on a certain point which will make your fish very unhappy to say the least and 2 as your ph rises, so will the Ammonia in your tank. At low ph, Ammonia starts to convert to ammonium for reasons unknown. Presumably witchcraft. Ammonium is not processed by your bio filter, but builds up all the same. As the ph rises, Ammonium converts back to Ammonia causing a spike that can kill your fish. So if you are already having issues with Ammonia, imagine what they will be like when your ph comes back up. In conclusion, fix current Ammonia problem, slowly add tap, monitor parameters, 50/50 tap to ro is a good mix to stick with. Lastly, keep us updated! I love nothing more than to actually see resolution to a problem.