I use RO water, and it's 99.99999999% pure. The conditionner removes chlorine and heavy metals (just the glamrock ones), so it's 100% unnecessary in my opinion. Conditionners like Aqua-Plus though cover fish with a protective membrane, which can be of interrest, but not worth it IMO.
The water is so pure that you have to add trace elements. If you use a buffer for an African Biotype or south american biotype, usually, you have those elements with it. If you have a biotype with a PH of 7.0, no buffer exists for you. trace elements can be availaible online but they never have some at my LFS, and looked at me like I was an alien when I asked for it. You can make a mix 90% RO Water and 10% conditionned tap water, and bingo! You have your trace elements. Some people think this method is useless though and say you polute your RO water, but I think that there's no panic!