The frequency with which an aquarium needs to be cleaned depends on various factors, including the size of the aquarium, the number of fish, the type of filtration and feeding habits. In general, a regular cleaning schedule should be followed to maintain water quality and ensure the health of the fish.

Partial water change: A partial water change of around 20-30% of the water should be carried out every 2–3 weeks to remove pollutants and improve the water quality. 

Cleaning the substrat
e: The substrate should be vacuumed regularly to remove excess food, fish droppings and dirt. This can be done at every water change or as required.

Filter maintenance: The filter should be cleaned regularly to remove deposits and dirt. Depending on the filter type, this can be done every 2–4 weeks.  Caution: Do not clean the filter and change the water at the same time.

Algae control: Algae can form in the aquarium and should be removed regularly, either by cleaning the surfaces or by using algae magnets or sponges. It is important to assess the cleaning requirements individually and adjust as necessary. 

Good observation of the fish and regular testing of the aquarium water will help to determine whether adjustments need to be made to the cleaning schedule. Translated with DeepL.com (free version)