[i have crabs]

you said you have no filter media correct?, so no filter socks/sponges/floss/bioballs anywhere in the system? how much rock do you have in the tank? pictures amuse us and it might help us see what your workin with.
sandbed? how old is the tank?
test the water your using for waterchanges and your tank water to compare the 2, under 20ppm nitrate isnt a death trap but can add to other issues, i dont think your tank is over stocked, the only time ive had lingering nitrates is when i employ some kind of mechanical filter since i tend not to keep them clean enough,i feed frozen mysis,reefroids,cyclopeeze daily, i rinse out the mysis and make icecubes with everything mixed in that way its ready to go and pre portioned, since i started doing this im much better at not over feeding, i never have any problems nitrates in my reef and i think i still feed quite abit over all. i have quite a bit of live rock and a decient sized skimmer but thats it for filtering. i do 25-30% change ever 4-6 weeks in my reef and its worked well.
i run carbon and gfo 24/7 but dont relate them to nitrates in anyway,