I use RO water in my marine tank, but wondered if there any benefits of using it to do my water changes on my freshwater tank? i know tap water is ok. but if candles are ok why invent the lightbulb!!
RO water on it's own is not suitable for a freshwater tank. It is almost completely unbuffered, (not the case in a marine tank), and can swing wildly in pH. You need to either mix it with some tapwater to acheive the desired chemistry, or add stabilising salts to it, Kent RO Right for example. You really need to understand your water chemistry.
If youve got it use it, RO water is far better than most tapwater which is often full of phosphates, high nitrates and god knows what additives. Just follow the instructions on the replacement minerals and it'll be fine.