Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
In any decision like this you need to look at what your end goal is and what tools and supplies you have to reach that goal.
If you want a soft water tank, and your tap water is hard, then adding RO to the tap water can be a very good answer. Use enough tap to supply most of the minerals, and enough RO to keep the GH and KH low. Add what you need to complete the parameters and keep them in the right range for the fish and plants.
If your tap water is toxic and you do not want to use it then starting with RO and adding the minerals as needed for fish and plants is a good way to go.
The answer for each person, each tank will be different.