I recently dirted my tank and right now I have some questions. I already capped it off with gravel. I don't like the way the gravel looks so I'm just gonna cap the gravel with black sand. I have a 20 gallon and i'm wondering if 5 pounds of black sand is enough to cover the tank. Or would I need 10 lbs? I'm not looking for a thick layer just a layer enough to cover the gravel.