Update: FW cleanings:

Pods eating the algae: The reason you need to run freshwater over your screen every week is because you want to kill the baby pods that start to grow and eat the algae. You may not see the eaten areas, unless the algae is very thin like this:



However, pods are always growing and multiplying. And they eat algae. The reason this is a problem is that (1) pods are constantly flowing out of your scrubber into the tank. If they eat algae first, then the nitrate and phosphate that is in that algae gets released back into the water, and (2) you now have less algae to do the filtering. Unlike the picture above, however, the algae is usually too thick for the eaten areas to be seen. The pods stay out of the light, in the underlying layers; so you don't see what they are eating. By cleaning your screen in freshwater, you kill the pods that are on your screen. They will start multiplying again within a few minutes, but at least you can keep them under 7 days old. And even if you clean only half of the screen each week, you still want to wash the whole screen in FW.