Of course, you have to clean and to change sometimes the water because in the water appears many painful bacterias for fishes. The aquarium has to be cleaned every month with a sterile usual sponge for dishes. Also, exist many types of additives that influence benefic for fishes. I'm buying from ARCREEF some live rocks and reefs for making more real the habitat of fishes. I recommend you watch some videos from youtube 'how to clean an aquarium'.
How to clean aquarium? Or is there a thing that cleans aquarium itself?
This is a pretty huge topic. Can you clarify more about the situation? Obviously this isn't an aquarium that you set up, so did you inherit it from someone? Did they share their maintenance regimen? Aquariums should have ongoing filtration, but still need additional regular cleaning to include wiping the glass and doing water changes.
Or are you asking about cleaning an empty aquarium?