If you get coarse sand that comes from the aquarium store that is coated with plastic and screened to remove the finer particles it should work. I would buy this sand and strain it through a screen over and over with water to remove whatever is left with a hose outside or something, and I would make sure there are no holes in the edges of the UG filter as well. If there are any holes in the edges of your filter it will get clogged. I know it can work because I've done it in my aquariums. The slits are small enough to keep the coarse sifted sand out and it's always the cracks in the edges and underneath the edges of the plates where the sand can get in. Where I've had problems is when I've moved the riser tubes around too much in order to get a big power-head to fit and it lifted up the back edges of the filter plates which let sand get in. It was very hard to get the filter plates to sit back down after I lifted them up, so start by making the plates sit perfectly flat (I would set up the filter plates on a flat table to see if there are no holes) and it should work. Sand will get clogged with debris faster than gravel so you'll have to clean it with a siphon type gravel cleaner more often, and buy a large siphon because the smaller ones just suck the sand up making it harder to do. Use your judgement on this. Like I've said, it's worked with me, but I can't guarantee it'll work for you. I would have at least 3 inches of sand so the weight can hold the filter plates down better, and I've had to pump water backwards down the riser tube sometimes to get the flow going again. Metallica's suggestion may be better in the long run because it is a bit of a pain and is probably esier to just run more filtration elswhere.Good luck!
