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

Thanks again.
I'm going to keep it simple and forget about trying to switch to spring water as questionable purity and consistency. He seems to be doing great with just the RO water plus electrolytes and the pH is stable at 6.8.
Oh, for reference, I tested out the Replenish but went back to the Kent RO Right as the Replenish has zero dKH. One has to add their carbonate buffer (Alkalinity) to establish KH. The Kent gives me a dKH of 3 out of the bottle. I'll experiment with the other product when I have a bit more experience under my belt.
Thanks again for putting up with all my questions. You and Thor are great source of knowledge and experience and it's greatly needed as starting out I've been given so much misinformation and bad advice.