Hello again friend. It is not advised to use a UGF with sand as you suspected, the sand is just sucked through it making it ineffective. I'll also put in my personal thoughts that the UGF is not the best filter out there. I used to use them but much prefer the filters that I can clean, maintain, and even check to see how they are doing. There are die-hard UGF fans out there and some tanks do great for years with a UGF but for me it's no longer an option... to many better systems out there.
ALASKA