Just a thought but you may not have enough water flow going through your sump, I have about 300 gph going through mine and it's on a 55 gal. With a 210 gal tank you need more water flow to the sump, I have a 180 gal with two overflows rated at 700 gal each so I'm cycling 1400 gph through my sump.
I had wondered about that recently, after seeing what everyone else is running (much larger). I hope to one day to build or buy a larger sump, possibly with a refugium. I hope that with what I have it will suffice for awhile. So far the water parameters haven't shown any problems...I just did my first routine water change. I assume that something would show up somewhere if the filter wasn't doing it's job, right? Please tell me if I'm wrong.Just a thought but you may not have enough water flow going through your sump, I have about 300 gph going through mine and it's on a 55 gal. With a 210 gal tank you need more water flow to the sump, I have a 180 gal with two overflows rated at 700 gal each so I'm cycling 1400 gph through my sump.