Q: I heard that you have to watch out for getting an "oil slick" on your screen.
A: "Oil slicks", or very dark or black algae, usually happen when a scrubber is first set up on a tank with very high nutrients. This algae does not get thick, and it doesn't even look like algae. The problem is when the person thinks that his screen is "just not growing yet", and he leaves the screen alone (not cleaning it) in order to "give it more time to grow. BIG MISTAKE. What actually is happening is that this particular type of algae is already fully-grown on the screen. And since it's very dark, the outer layers block ALL the light from the inner layers, even though the algae is only 1/4 inch (6mm) thick). So the inner layers die and go back into the water, adding nutrients back. So, the person leaves the screen alone even longer "to give it even more time to grow", but the new outer layers once again kill the inner layers. The screen never gets "thick" like the person wants, and so the person never cleans it. Nutrient levels stay the same in the tank, and the person feels that the scrubber is not working at all. The solution is to clean any and all dark/thin algae off as soon as it grows, EVEN IF it's just two days old. This will bring nutrients down, and after a few weeks of doing this, green hair algae will start to grow. At this point you can switch to normal weekly cleaning.