Why is filtration necessary?

Nature sets the right example

Every aquarium is a microcosm in its own right. As simple as it may appear at first glance, an aquarium is actually subject to the same complex laws of nature as prevail in such a vast body of water as the Amazon.
An understanding of the biological processes involved is crucial for a successfully functioning aquarium. The fish will then remain healthy, the plants will flourish, the water will be brilliantly clear and troublesome algae will remain in the background. Filtration plays a key role here. In nature, water undergoes continuous slow filtration and is continually replenished. It seeps through different strata of the earth and rocks. Gravel and sand in the beds of natural bodies of water are colonised by countless mircoorganisms and act like giant filters. Fish excrement is biodegraded here by bacteria. Microorganisms break down and remineralise dead plant matter. Suspended matter settles at points where the water flows more gently. The water which trickles or flows away is replaced by rainwater.