Here is a diagram by Eric Borneman that shows what feeds on what:



It was taken from Eric's two building block articles that cover what happens when you feed your tank. This information is what you need to know to really understand what scrubbers do:

http://reefkeeping.com/issues/2003-01/eb/index.php
http://reefkeeping.com/issues/2003-03/eb/index.php
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