We do not clean pond or lakes, but an aquarium needs to be cleaned. Why?

In the Aquarium, the uneaten food as well as the waste generated by the fish mixes with the water and is left untreated due to the lack of decomposers. The waste materials thus accumulate in the water making it toxic. Hence an aquarium has to be cleaned after regular intervals.

A pond or a lake ecosystem is a natural ecosystem and the types of organism present in it is balanced. The waste produced in this ecosystem serves as the food material for the decomposers and is thus eliminated by them from the water. This ensures a balance in the water body and keeps the water clean and thus does not require physical cleaning.