Carl ---- Ben did tell me that some filters do have gravel at the bottom. I'm not sure which do and which don't, though. My sand filter doesn't.

Vickeybear -- as far as your sand filter needing shelter from the sun, it isn't required. Mine has never been sheltered and it is doing just fine after 10 years. Of course, it wouldn't hurt it to cover it, but not needed. By the way, welcome to the forum!