I might try running a quality GAC
for a week or two, changing it every 5-7 days, to see if that has any effect. FWIW I'm not a big fan of HOB
filters. They tend to produce micro-bubbles and mechanical filtration seems to cause more issues than it solves in SW
-especially on larger tanks. Pads, floss etc. all trap dirt and detritus too fast. That can lead to water clarity issues as well as nitrate issues. Powerheads, LR
, a skimmer, and some GAC
are the way to go. Just my .02 ...