If you understand aquarium nitrogen cycle, you should know that the end product of the cycle - nitrate build up in the aquarium. Eventually the water buffer will be eaten away, and the PH will become lower and lower.
Weekly partial water change is recommended to remove nitrate, restore water buffer, as well as to add the necessary minerals back into the water to ensure a healthy aquarium system and fish. There is also the need to remove fish waste.
To understand the necessarily of weekly aquarium maintenance, check out this article in our fish library. Aquarium Maintenance
The most important thing is that you need to do (partial) water change. We typically do 30~50% partial water change once a week. Some people might do it more often while some others do it less often. Weekly is good enough unless the aquarium is too heavily stocked.
