It's a bad idea to use only RO water, if you are not reconstituting it. Fish and plants need the minerals etc. that are usually present in tap water. But if your tap water is not usable, you can reconstitute the RO water, but you have to be much more careful with water changes so as to make sure you measure very carefully each time what you are adding. My tap water is close to RO....0KH and 0GH...and when reconstituting the GH for my plants I have to carefully measure what I add....a few times I added too much when doing a 50% water change and ended up with fish deaths as a result. Now I only do 25% water changes to be safe, and I'm more careful when I add the GH. Mixing RO and tap water to soften it is fine, as long as you measure correctly, each and every time.