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

to the forum.

Don't mess with water PH. Do not use any product for PH altering. It will bring in more harm than good.
RO water has 0 buffer, any change will instantly crash the PH which is deadly to the fish. Fish like stable water conditions including PH. What you need is to add water buffer. So the PH will not crash easily.

If I am not mistaken, one of the products for adding buffer into water is Tetra Easybalance Plus. Check the product description for more details.

You do not need to maintain PH 7 in order to cycle an aquarium. As long as the PH isn't too low (too acidic. ex: PH 4 or lower), the bacteria will multiply just fine.

Hope it is helpful.