The breakdown of fish waste, too much food and dead plant matter will cause toxic compounds – namely ammonia - to build up in the water and kill your fish.
As an aquarium is a ‘closed’ environment it means harmful waste can build up quickly because there is not enough water to dilute it or to wash it away, as would be in a pond or river.
Thankfully, there is a natural process called the ‘Nitrogen Cycle’, which we can recreate in the aquarium, to convert the toxic waste into safer compounds (The image below explains how this process works).