Can not beat high nitrates 80ppm!

I setup a 72 gallon tank with a 20 gallon sump about 6 months ago and since about week 2 I have had high nitrates! How do I lower these?! I have a reef octopus nwb110 skimming wet, use 0tds RODI water for water changes and top off and I do 20% water changes weekly siphon the sand bed during water changes. In the sump I have nothing but my skimmer, micro algae, and return pump, and heaters. What else can I do to lower the nitrates? add a filter sock? or do different water changes? But from what I read you cant lower nitrates through water changes. I feed a 3x3 section of nori every other day, and a half cube of mysis everyday and Im careful not to give more than they can eat at a time. Live stock is a pair of clownfish, blue eye kole tang, and three anthia's My nitrates are well over 80ppm. any help will be appreciated! thank you guys!