well you can do a custom white balance by turning it to that setting and basically calibrate it to the sand. Sometimes when i do this, it makes the rest of the tank like a greenish tint. I dont know what kind of settings that camera has, but if it has an AV or aperture priority, that usually works pretty well if you set it as wide as it can get. Bump the iso to about 200 or so, so that its not too grainy.
If there are no manual settings, just turn the flash off, bump ISO up a bit more and keep the camera absolutely as still as you can.
Hope this helps