Once you have established a colony of beneficial bacteria you do not want to harm it. Cleaning your aquarium is essential to the good health of your fish but keeping all of those good bacteria is just as important. This is why it is essential to do partial water changes. Keeping some old water in the tank at all times will give the bacteria that are still present a chance to multiply and replenish. Full water changes will wipe out too much of the bacteria which may allow a spike in ammonia and nitrite levels resulting in possible death. Another important thing to remember is not to use any cleaning chemicals that do not specify safe for use in aquariums. Bacteria are very sensitive to chemicals and you would not want to kill your colony of beneficial bacteria. If you need to clean any filter cartridges, it is a good idea to rinse them in the water removed from the aquarium because this water already contains the bacteria.
Basic Water Chemistry Part 3: Ammonia, Nitrites and ..
A water change, however, will remove some of the excess ammoniafrom the aquarium (provided you are not using with a lot of ammonia init), and will allow the biological filter to begin to process theexcess waste while relieving the stress on the fish.
Using an Ammonia Remover/Dechlorinator | DrTim's Aquatics
Your aquatic ecosystem depends on relatively steady water chemistry. The nitrate levels, along with most other aspects of the water makeup, need to remain consistent for the health of everything living in it. Sudden considerable changes to any readings -- from temperature to ammonia to carbon dioxide to nitrates and other levels -- stress tank life and can be fatal when severe or when particularly sensitive species are present. Employ only one effort to remove high nitrates per day, or wait a few day between tactics if your levels aren't dangerously elevated, to bring down levels gradually and avoid shocking anyone or anything living in the aquarium.
Reducing Ammonia Toxicity in a Saltwater Aquarium - The Spruce