remove the fluval. these filters are great on a freshwater setup but on a marine tank they do more harm then good.
im going to guess that the filter has built up excess nutrients which caused your high nitrate issues. i need more information on the tank, saying your water is "good" tells me nothing. what exactly is "good?" please post your exact water parameters, ammonia, nitrite, nitrate, pH, temp, salinity ( and what you used to test it ) mag, ca, and alk. if you dont have your own test kits for these i strongly suggest picking up these. liquid kits are far more accurate then strips.
what livestock, other filtration? skimmer?(brand/model) sump? flow? lights? depth of substrate? kind of substrate? how long has the tank been set up? and everything and anything else you can include.
welcome to the forum.