RO water and KH problem with new populated tank

I started an aquarium on Dec 14th. The water in my home is not
suitable for an aquarium so I decided to use RO water. I knew I had to
add a conditioner to the water so I put Kent Marine RO Right in the
water. What I didn't know is that I had to adjust the KH of the water
to prevent ph crashes, and sure enough I had a low ph problem which
actually caused an ammonia problem. I have been adding ph Stable to
the water to increase the KH, but it seems to be stuck at 0. I have 10
neon tetras in a 20 gallon tank and can't just dump in alot of
chemicals or the fish will die. Now my ph is about 6.8 and
ammonia/nitrite zero, but I need to get the KH up.

Is there a way that I can increase the KH without driving up the ph
real fast ? I don't want the ph to increase more than about .2 per day,
and every time I add ph Stable the kh stays the same but the ph rises.
I have also tried about 2 tbsp of crushed coral, but that rose my ph
too fast so I took it out. The kh did get to about 1.0 after that, but
it has dropped again.