Big Al's sells Tahitian black sand, not sure if you have a local store in your area. Capping with black sand though, all that is going to happen is the sand is going to makes it way down through the fluorite and settle on the bottom eventually.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.
Sump Pimp #2
My Tanks and my shrimps