Remineralizing RO Water Additives

Hello! I'm new to keeping discus and will hopefully get discus at the beginning of the year after my 125 gallon aquarium is finished with its fishless cycle. My fiance is pretty adamant about getting an RO water for me on black Friday and doesn't want me to use tap water. I currently have discus trace from seachem and I have 4 55 gallon water barrels that I can use to prepare water. What else do I need to remineralize my RO water? Preferably products that I can buy in the US.

you can use water purifier device which is being sold on amazon or ebay

2 player games

You can just do a 50/50 or a 25/75 mix with tap water.