Silicone! I used tiles before in my 120 amd didnt silicone them in. At first i thought it wouldnt matter as long as they were cut to size and fit nicely. It worked.for a few months, but eventually gunk kept getting under the tiles and i had to lift each one during waterchanges to clean under them. Tiles are great! I personally would recommemd them over a bare bottom, especially since theres soo many decorative styles to choose from. But sealing them in place is a must!