We do not clean ponds or lakes but an aquarium needs to be cleaned periodically. Why?

Ponds and lakes are complete ecosystems, they have decomposers which decompose the dead bodies of plants and animals and thus keep the ponds and lakes clean automatically.

Aquarium does not have decomposers to clean the dead remains of fishes, or the aquatic organisms; so they need to be cleaned periodically in order to remove the dead matter pile up.