You’ve seen it: that shimmering, iridescent film floating on the top of your water. It looks like an oil slick, but you haven't put any oil in your tank. For many freshwater keepers, this "surface scum" is a frustrating mystery that ruins the view of a beautiful aquascape. While it might just look like an aesthetic nuisance, surface scum—scientifically known as biofilm —is actually a sign of a biological imbalance that can have serious consequences for your fish and plants. T