Question

We donot clean ponds or lakes but an aquarium needs to be cleaned. Why?

Answer 1

because many people come there to see aquarium .so it should be cleaned

Answer 2

It is because ponds or lakes are natural ecosysytems which have microorganisms which act as cleansing agent while aquarium is an artificial ecosystem having no microorganism which can clean it.That's why we need to clean an aquarium but not a pond or lake.
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