An aquarium is a closed system, so dirt and waste remain in the aquarium, turning it into a poisonous sewer without our intervention. This is why a filter is one of the most essential pieces of equipment in your fish tank.
It acts like a mini sewerage treatment works, and cleans up the aquarium water in several ways:
Your filter will remove dirt particles from the water. Cleaning you filter every month removes the dirt to keep things running smoothly.
Friendly bacteria - the majority of which live in the biological media in your filter - processes waste compounds to stop them poisoning the fish. See – Understanding the Nitrogen cycle.
Your filter will remove algae food, harmful metals, toxins, nasty odours and organic dyes for crystal clear, healthy water.
Your filter will move water around the tank to help add essential oxygen and remove carbon dioxide.