Excerpt from "Waste Extraction, the Invertibrate Way" by Ron Shimek
http://reefkeeping.com/issues/2005-08/rs/index.php

"The animal poops it out, and from then on the scavengers/detritivores get rid of it." This is, of course, a very concise way of thinking about the elimination of uneaten food from the digestive tract. Unfortunately, it has nothing at all to do with what biologists consider to be waste. Not to put too fine a point on it, but fecal matter is nothing more than uneaten, partially digested and processed food.

"Actual waste materials are something else altogether. Strictly speaking, to a biologist, only a couple types of materials are truly waste materials. These are the byproducts of cellular respiration and protein metabolism, which in most animals, are carbon dioxide and ammonia, respectively.

[scrubbers remove ammonia and carbon dioxide; skimmers do not]