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

Explanation

A pond or lake is a natural ecosystem. They are self-sustaining and complete, in then, all the organisms of food chain are available. If any organism dies, there are decomposers to decompose their bodies into simpler substances.
An aquarium, on the contrary is an artificial and incomplete ecosystem. The abiotic components are not supplied naturaly to it. It may not have all the biotic components in it. If a fish die in an aquarium in the absence of decomposer, it will lie there as a rotten body, polluting the water of aquarium. Thus, an aquarium needs regular cleaning.