It's a new tank I've set up; it only has sand and water right now. I'm using a sand I've never used before, and I didn't rinse it out before putting it in. I noticed that the water didn't clear up in just a few days after I added the water and sand, so I'm thinking this sand had really small dust particles or something that doesn't sink very well.
I may have to drain the tank, wash the sand, and then refill it.