● An aquarium is an artificial ecosystem. In an aquarium organisms live in a very limited space and restricted environment. Uneaten food as well as the waste excreted by the organisms mix 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.

● On the other hand, a pond or a lake ecosystem is a natural ecosystem covering larger area. The waste produced in lake or pond gets decomposed by microorganisms. This ensures a balance in the water body and keeps the water clean and thus does not require physical cleaning.