As opposed to mineral deposits fogging up the walls of your reservoir, surface scum is a visible film of micro organisms that floats on the surface of the water. Also called biofilm, it thrives in the transition layer between air and water. The scum layer may include algae, bacteria, and fungi and it's a real problem. These microorganisms consume the same nutrients your plants do, including oxygen, and they prevent gas exchange between the water and air. They're competing directly with your plants while they can also clog equipment like pumps and spray nozzles.
The best way to prevent scum buildup is to limit the amount of light that reaches your reservoir and keep the reservoir on the cool side, either by locating it in a cool room or adding a chiller. Light exposure can be managed with the addition of an opaque lid or even dark plastic. If scum has already become a problem, you'll need to clean your reservoir in addition to adding these preventive measures. Both hydrogen peroxide and bleach are effective at cleaning up scum, but make sure you've thoroughly rinsed all the parts before you start your next crop.
