A major concern for the aquarium with sand as the substrate is anaerobic gas pockets. The combination of the fine grains of sand and the depth of your substrate can lead to these pockets that are actually hydrogen sulfide. Hydrogen Sulfide, often seen as H2S, is toxic to you (large amounts) and to your fish! There are several ways to help combat this potential problem. The first would be, as with gravel, don’t go to deep. A very fine gravel should not be more than 3 cm deep and allowing more depth as the grain size increases. As a fish keeper you can help prevent these pockets when you do your routine maintenance by simply stirring the sand with a finger or other similar aquascaping tool.
Here's a great link that talks about H2S including Steve Hampton and Tom Barr:
http://www.plantgeek.net/forum/viewtopic.php?t=1424