This is geared for the newbie. The first thing to remember about cleaning an aquarium is not to go overboard. In reality aquariums are mini sewer systems. The fish put waste into the water both solid and liquid form. This waste break down to produce or is ammonia. I will briefly cover the nitrogen cycle. All over the planet earth organisms are producing ammonia. This is made when proteins are broken down. Ammonia in high enough concentration is toxic to many living organisms. There are bacteria that feed on ammonia and oxidize it to produce nitrite or NO2. This is also a toxic substance to many organisms. There is another type of bacteria that take NO2 and produce Nitrate or NO3. This is not very toxic. Many plants can use either ammonia or nitrate as plant food. There are many aquarium plants that will absorb ammonia directly. Also algae will do this. A cause of algae blooms (rapid increase in the number of algae) is excess fish waste. Live plants will help keep the algae in control when done with regular water changes.

It is hard to keep a closed system that is perfectly balanced. The planet earth is the only working model. When you change the water is are getting rid of the excess nitrates. Also if the water is evaporating from your tank there will be a slow build up of minerals and salts over time.

When are vacuuming the gravel you should only clean one third to half of the gravel at any one time. You want to leave the other side undisturbed to provide a source for more bacteria to colonize the areas that you just cleaned. One of the best biggest mistakes that newbie make is to clean the whole tank at once. The ammonia level might spike and kill some or all of your fish.

You also do not want to change all the water in the aquarium at once. It is important to use de-chlorinator when adding tap water to the aquarium. Most of the tap water in the country contains either chlorine or chloramine. Chloramine is a compound containing both chlorine and ammonia. Both can kill fish. It is also best to match the temperature as near as possible. Some fish are sensitive to sudden temperature change. In their nature environments the temperature changes slowly over hours or days. Salt water fish are even less forgiving since the ocean never changes temperature suddenly.

Water changes are important for fish health. All aquarium should have a partial water change every week. I would say that a 20% to 33% water change is a good thing. Most of the problems that cause fish diseases are related or be cured my changing the water. Fish living in clean water are healthier.