200+ ppm nitrates. help

i have a 150 gal fish only tank with super nitrates. 200 plus pm ive been doing big water changes and nothing will budge. today i decided to go all out and took apart the sump to get all the detritus and washed it out, i moved all the live rock siphoned it all up and did a 50 gallon water change but still 200 plus ppm? why? anyone else