Question

We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Explain.

Answer 1

Ponds and lakes are natural ecosystems. They have decomposers or natural cleaners to decompose the wastes of all aquatic organisms. So it  is not necessary to clean it.
But aquariums are man made ecosystems. They dont have decomposers or natural cleaners. So it should be cleaned regularly....

Answer 2

We do not clean ponds or lakes , but an aquarium needs to be cleaned. ... In the Aquarium the uneaten food as well as the waste generated by the fishes mixes with the water and is left untreated due to the lack of decomposers. The waste materials thus accumulate in the water making it toxic.