Greg Dalton wrote:

Setting up a plant tank(90G)...

I was curious about using some rock that I've had for 10 years(in
cichlid tanks)...

Its cave rock. My LFS has highly recommended that I NOT use it in the
plant tank because the water would leach minerals and stuff out of it
and harden my already hard water. I used a large portion of RO water to
soften my water a bit and the LFS store guy added that it would increase
the leaching even more...

Your fish store guy seems to be under the impression that hard water is bad
for plants. This is not the case. Although some of us choose to have soft
water tanks for SOFT WATER FISH, the vast majority of aquatic plants do
better in hard water than soft.

What are your water parameters?