RO water out of the tap, what are best products for conditioning and restoring kh?

Thanks Thor!
Followed your advice on water conditioning my RO water and my betta is doing great! He had some tears and holes in his tail fin when I brought him home and all are healed as of this week and his activity level is high and his fins are no longer clamped. Great appetite (I am following your advice from the beginner's guide and feeding small amounts once per day) and he just started building a bubble nest yesterday and he flares regularly. It's a joy to watch him dance around his tank.
I have some questions about fishless cycling but I will not ask them on this thread.

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Thanks RAM for your valuable input as well.