what do you put in the bottom of your tank?

So far, we have used nothing on the bottom of our tank. It’s just plain glass. Our dbt looks like she is slip sliding away sometimes (she does have some bricks for sunbathing). Is there something that all of you use on the bottom of your tanks (like rocks) or do you leave them bare, too? We’d like to make her home comfy,but we don’t want it to be a nightmare to clean. Rowena P.S.: when she lives outdoors, she has bark mulch substrate and a very big cement mixing tub with a smooth surface bottom.