The reason for adding a filter to the fish tank is to get rid of toxins, odors and discoloration in water, including nitrates and ammonia. We also want to add oxygen to the water.
What is filtration?
Filtration is the general cleansing process of a contained body of water.
In nature filtration is accomplished in several ways. For example in a body of water the current will steadily carry unwanted materials downstream. Large amounts of other life forms occupying the waters also help ensure lakes and rivers remain clean.
Unlike natural waters, a fish tank or pond is smaller in size and has no means to rid itself of unwanted materials and chemicals. Filters do this job.