Hurray for planted goldfish tanks!
I really thought about using Miracle Gro Organic under my sand, but the stuff I bought wouldn't sink in water, even after having it in a test bowl for over a week. I would scoop out whatever was floating, and the next day it was all gross again. Even with a sand cap, I think it would have made a mess. I couldn't find anyone having the same problem, so I figured I had a bad batch, but decided to stick with root tabs anyway. I'll have to give my next tank a better plant foundation.
I bet your goldfish love their home!