Question

We do not clean ponds and lake but an aquarium needs to clean.why?

Answer 1

Brother this is because aquarium does not contain soil and decomposing bacteria which are used in degrading complex organic substance into simple organic substance and act as a cleaner. But pond have this facility.
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Answer 2

$\large\tt\orange{Answer☞}$

An aquarium is an artificial ecosystem which is incomplete ecosystem when compared to pond or lake which is a natural and complete ecosystem.