A good start for WB is to set your camera to custom WB (PRE). If you have a Nikon it likely looks like this on the top:
Hold down the WB on the left and spin the thumb dial on the right until the little "A" goes to "PRE". Hold down the WB button until it blinks "PRE" in the right window. You are now ready to take a picture of something that is near neutral grey in your tank. Make sure the lights in your tank are all on, and snap a picture of the blandest part of your live rock, white or grey it doesn't really matter. If the camera accepts the WB, it will blink "GooD", if it doesn't try again. Just try to fill the frame with the rock work. Now shoot away in your tank and the WB should at least be better than on "A" setting.
If you don't want to do all that you can try the cloudy WB setting or dial it up to 10,000K in the manual WB mode.
Have fun!