I have had a 60G FOWLR with a 17G sump set up for about 6 months. I am running a Deltec MC500 skimmer, 2x Sicce Voyager 4 power heads, 2x Eheim 2000 compact return pumps, one through a Resun CL-450 chiller. I change 10% RODI water and the 200 micron filter sock weekly and still have Nitrates at about 15-20ppm and recently inherited a Cyano slick! Could it be the 200 bio balls in the sump?