you "can" use RO/DI water, but not necessary unless you have unusually hard tap water.
If you want to use it, just mix it with tap water to a point where get to 3 dKH and 7 dGH (obviously, both may not be attainable, but you can correct it as necessary afterwards). Unless one of those goes below 3 or 7 in the meantime, you will not need to add anything else. Baking soda will increase KH, and Calcium/Magnesium will increase GH.
Post what your current tap water KH and GH are and we can help you meet that proportion of RO/tap.
I thought so originally as well. I have a 6-stage RO/DI/Ammonia Buster unit in my garage. Only use it for drinking water now
The main difference also between using RO for FW vs SW, is that in FW you don't have crushed coral, shells, regular coral, etc, contributing to replenishing GH and KH levels. If you were to just toss in straight RO water you would soon have dead fish.
You need to buy some RO Right and use baking soda to get to an acceptable GH and KH level.