Originally Posted by ArtesiaWells
Are you referring to some algae buildup within the plastic pieces of the filter, or just bacteria that eventually latches on from uneaten food, waste, etc?
Most filters, over time, develop a film of sludge that can coat the internal walls and act as a lubrication. Too much of it is not healthy for the tank and will eventually impede the filter's performance. All of my filters are AquaClear filters and I've noticed that when I run them right out of the box they are usually louder and minute bubbles come out of the output. After a while, say, 30 to 45 days, they run quieter and the bubbles seem to disappear. I can only attribute that to the film that coats the internal walls and mechanism.
I keep all my filters on the plus side. I want water turnover to be at its max setting. The more water that I can run through the biological media per hour, the more nitrifying will take place. However, depending on what I'm feeding, I may decrease the water flow. Sometimes I even shut the filter off; it's connected to a socket that's hooked up to a wall switch (it's like turning a light on and off).
Pick an area in your tank where the water flow is at a minimum and feed your fish in that area every day. Your fish will eventually learn where they are to be fed and will meet you at that spot before you get to the tank.