Natural water bodies are not regularly cleaned whereas an aquarium needs regular cleaning. Why?
Lakes and ponds are natural ecosystems and contain bacteria and diverse organisms to decompose organic substances. The waste in these ecosystems is food for some organisms living in it and so it is consumed in the water body itself. This keeps the water of ponds and lakes clean and so they do not need regular manual cleaning. On the other hand, aquariums are artificial ecosystems and do not contain many aquatic organisms. They are also comparatively small in their area. Hence, they require regular cleaning.
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