Question

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

Answer 1

A pond or a lake has its own natural system of cleaning the waste materials to maintain its ecosystem. Due to this, ponds and lakes normally do not need to be cleaned.
An aquarium is a man-made ecosystem which means biological community living in it rely on humans for their food and other physical need. Along with their dependency on food and other physical needs, an aquarium also do not clean itself but rely on humans.