using RO water

Hi Guys
I'm using R.O water with Tap water because of high nitrate's in the tap water 20ppm.

Lightly Planted freshwater aquarium with some small goldfish.

This will lower the KH & PH, I want to keep both the KH & PH stable so I will need to buffer the water.
Is there a safe and natural way to do this?
I have read about crushed coral and pumice stone and also backing soda! Which is best for stability and how easy are these to get.

Thanks
Obliged
Paul B

I've used crushed coral and coral sand in the past, it's slow to react and is very stable. I'd imagine baking soda would be instant and may cause a shock to both fish and water chemistry. Slow and steady is the way to go.

Thanks Paul
Can I get these in local pet shops (petstop/maxi zoo) or just online

I got mine in aquatic village. I'm sure sea horse has it. The pet shop chains may not stock it but could order it. It's heavy so online postage could be expensive. Make a few phone calls.