A BALANCED AQUARIUM
Although an aquarium constitutes a self-contained environment, it nevertheless
evolves: fish get bigger and reproduce, while plants grow. Similarly, the water
undergoes changes that may not always be visible - the nitrate concentration increases
but the mineral salts depleate- which is why it is important to analyze it regularly.
As for the bed, this is enriched by numerous organic substances (animal excreta,
surplus food) that will normally be converted into mineral elements.