Hey, so this is what I know from my research. Tiles such as slate and ceramic are good options. However, you must consider that they are slightly porous and will hold bacteria. You need to clean and rinse them very thoroughly to make sure no bacteria or cleaning chemicals remain. (if you use a bleach water solution rise really well with hot water until no chemical smell is present). These tiles also tend to be more expensive and heavier. So if you have a large tank, the weight and price may be an issue. Also, unless you are able to buy tiles that will fit your tank exactly, you will have to figure out how to cut the tiles to appropriate size. Another option is linoleum or vinyl. This is what I use. The nice thing about these tiles is that they are much cheaper, lighter weight, and non porous. You still need to spot clean and do a throughout cleaning once a week though. I purchased individual 12x12 tiles at my local hardware store and cut them with scissors to fit my tank. They are adhesive on the back so they stick to the bottom. The only problem I have run into is that the adhesive makes it hard to remove and clean the tiles completely. When I do my throughout cleaning, I do not like to leave them in there because I worry that any chemicals I am using will get trapped under the tiles or ruin the adhesive. I haven't found a great solution for this problem yet. As for making tiles less slippery, look for ones that have some texture and grab. Unfortunately, a lot of finished tiles are smooth. You can either try a more natural stone like slate with a little more grit, or get some basking rocks that will provide traction and grind down the nails. I hope this helped a little bit!
Best of luck,
Jen