Okay, I just read an article on this - Freshwater and Marine Aquarium, Feb 2007.
One quick quote: "What they didn't know was that silicone doesn't stick to slate for very long."
They recommend using a glass bottom instead of the slate or lining the slate bottom with a glass liner that's all siliconed up - so you essentially have an all-glass aquarium on the inside with the slate liner on the outside bottom.
The article is called "Remanufacturing Metal-Frame Aquariums"
I could probably be convinced to mail it to you when I'm done with the mag...