You will need to remineralize it, what is your tap water like? In my case I use 70% ro/di 30% well water and that gives me a ph of 7, gh of 5 and kh of 3. Lots of people use tap depending where they are from.
I use 100% Ro for my planted tank. Never had any issues. I used to do 100% RO/DI but got tired of remineralizing it.
I am guessing you have a smaller tank with an active substrate. Also dosing EI with gh booster will help, but I guess you will quickly deplete your active substrate. On a large tank you will have issues if you use only ro/di unless you dose gh booster and some sort of k2co3 to increase your kh (clearly this is dependent on the substrate)