I use BRS for a RO unit. http://www.bulkreefsupply.com/bulk-r...s-systems.html
Their site is super helpful with videos and all on their equipment.
If you mix tap and RO then you should not need to remineralize.
The tap is adding back what will be offered without knowing the exact makeup of water and knowing what is needed to match the fishes needs.
If you know what the fish needs and how to make it then you start 100% RO and remineralize..
This is the best post I have found to date on how to really /properly remineralize ... RODI balancing - The Planted Tank Forum