You can easily add more sand. I recently added 20 pounds to my 135g. The way to do this is as follows: Purchase (or steal) high quality sand designed for saltwater tanks. Rinse it out with filtered, R/O water. Some will say you can use tap water to rinse it, I would not do this. Then, once your rinsed water is no longer foggy, you can wait for the sand to dry (which will take forever), or you don't have to wait, and use a long piece of PVC
pipe. Place the pipe upright in the aquarium. Using a funnel, pour the sand into the pipe and softly direct where the sand goes at the bottom of the tank. This prevents all the problems associated with sand being blown all over the tank, the resulting sand fog that blacks out your tank for a few hours and the time it takes to clear off sand from your corals and rock.