i agree with jt, the sand would probably be more trouble than it is worth. the reason i say that is that you have to have enough area for the sand dwelling critters to thrive for the sand bed to be truly effective. it doesnt sound like that would be the case in your application. i would just add some lr rubble. just remember, every time you have to take the skimmer out to clean it, you will likely have to deal with moving the rubble out of the way to put the skimmer back in.