15-20 is too small.
Should be 40g breeder, or at minimum 29 gallon standard. What's important is the foot print of the bottom, you want it to be close to 36" in width, but 30" can work as well.
They also need a u-shaped burrow with 2 entrances, this can be simulated with PVC or just really well placed rock (preferably the later, as long as its completely covered and dark), they need to be able to use rubble to close up both entrances when molting happens, you can sledge hammer large pieces of rock and drop the rubble in the back, letting it pick up and carry the pieces it wants and then use snail/clam shells to supplement even more building material. If they can't close up their burrow or have the u-shape setup they will stress out and die during molts, and if they're too exposed to light they can develop shell disease (if you ain't running an SPS reef sterile setup I wouldn't recommend strong lighting at all, you can get away with it if the water quality is prestine).
1" sand is enough, but I usually aim for 2, mine never exposed glass at the bottom.
Its also advisable to lay a sheet of acrylic/pexi on the bottom of the tank to prevent any burrowing down glass breaks.
With the setup you described you're probably better off with an N. wennerae, G. smithii, P. ciliata, or G. chiragra to name a few.
