Generally if you are going to use RO/DI in a FW system you must restore the Ca, Mg and so on that was removed by the RO/DI process. You can use a commercial produce such as Ro Right to do this. Typically you want to take GH and KH to about 4 degrees for very soft water.
This may be a whole lot more trouble than it's worth, especially for a FO system. Test your tap water first and only use RO/DI water as a base if you have a specific problem with your tap water or are keeping extremely demanding fish such as discus.
If you have very hard water, and no other issues, you can use a mixture of tap water and RO/DI water to get the hardness you want.