Question

why does an aquarium need to have cleaned once a while? do we need to clean the ponds too in the same manner? why? why not?

Answer 1

It is because at regular intervals the bacteria and the other harmful chemicals that generate in the water of the aquarium form algae which is very harmful for fish in aquarium. To get rid of the algae, we must clean the aquarium once a while.
In ponds as well, algae should be periodically weeded out for the proper growth of fish.