The reason we have to clean a fish tank but not a pond is thatPonds and lakes being natural ecosystems have natural decomposers which are cleaners.
Why should tanks be cleaned but not ponds?
Lakes and ponds are natural environments. They include a variety of decomposers that serve as purifying agents. By comparison, the aquarium is a man-made environment. There are no such decomposers in it.
Since ponds and lakes are natural ecosystems, they naturally include cleaners and decomposers; as a result, we do not need to clean them. Aquariums are man-made habitats that typically lack certain elements of a real ecosystem.