If you have surface scum that might have more to do with a poorly functioning overflow. What's the current setup and how many gallons is the return pump?Has anyone ever ran two mp40s on the same side of the tank facing the overflow? I am having issues with sediments sitting on the top of the water and only being pushed around. I have a 80gallon 4ft shallow tank. I was wondering if I put both on the same side if it would eliminate all of the floating stuff on the top of the water.