CBSE Class 10 Science 2019 · Paper 31/3/3 · Question 2
Generally, we do not clean natural ponds or lakes but an aquarium needs periodic cleaning. Why ?
Answer
A natural pond is self-sustaining because decomposers clean it, whereas an aquarium lacks enough decomposers, so wastes pile up.
A natural pond or lake is a complete, self-sustaining ecosystem: its decomposers (bacteria and fungi) break down wastes and dead remains and return the nutrients to the water, so it cleans itself. An aquarium is an artificial ecosystem that lacks enough decomposers, so wastes, uneaten food and dead matter pile up in it; hence it needs periodic cleaning.
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