This is my issue with my Red Sea rsk300 on my 75/20l sump setup. Nothing but weak tea colored water. I was actually thinking of going back to my Reef Octo hob 1000. Blasphemy I know but it produced nice thick dark skim mate for me in this tank. It's to tall for my sump and I'd have to extend the pickup and return tubes or I'd give it a try. I now have the rsk300 on a timer only running 6 hours a day. Had to put my ato on an opposite schedule to account for water volume in sump skimmer running vs not running and all worked out now. Still just getting the tea colored water like this but at least now I have a nice thick dark sludge coat on the cup neck and the Red Sea neck wiper panel thing.light bioload? maybe there's nothing to pull?