Aquariums are in essence a “closed system” – that is to say that when your fish defecate or food is left uneaten, there is simply nowhere for it go apart from floating around in the water. For obvious reasons, over time this build-up of organic waste can be harmful for your fish, as well as making your aquarium unsightly.
The secret to maintaining a clean aquarium is “little and often”; this means that it is recommended to clean the tank regularly during the week, instead of leaving several weeks without cleaning the fish tank.
This is recommended since fish are very sensitive animals, and very sudden changes in the environment in which they are found, such as temperature or water chemistry, have a great impact and can even be fatal to them. For example, it is better to replace small amounts of water, than to empty the tank completely, as this will avoid damaging essential bacteria that are in the tank and contribute to the cleaning of the water.
Read full article in PBS Specialist Pet Travel