I would try twin spot goby, they sift sand but only a little :-)
Also the danger of having too thick of sand is that as detritus and organic material make its way down you can get pockets of hydrogen sulphide, or H2S, it is extremely poisonous and would crash a tank within a day or two. I would argue max 2 inches (or 2 finger rule) of sand to be on the safe side.