I had some major red slime algae in my sump in the fuge section where I run a light on an alternate photoperiod from my tank, but NO algae at all in my tank. The only difference I'd say between the two is wattage per gal. Before upgrading my lights earlier this week I had 1.4 watts per gallon in the tank and 3.6 watts per gal on my sump. Just some thoughts I'm throwing out. Maybe lighting has something to do with it. Not sure about flow but I have about 750 gph pumping through a 12 gal sump, and in the tank, I have 2670 gph. My nitrates are at 6ppm right now. Your tank may still be cycling though which means the algae will come and go for a bit. I guess I never got it in the tank because the conditions were better for it in the sump, but nevertheless it was still there. No 2 systems are the same though. I just wanted to give you an idea of what is happening in my tank (5 months old) incase it may be of some help to you or spark some thought for you...Best of luck daytonabill!