Water changes....we all do them, or should be doing them. Yet, are they being done for the benefit of the animals, or for the convenience of the hobbyist? Water changes involve more than just removing water from the tank, as we shall see.
Water changes are an essential part of maintaining the quality of the captive environment. Without them, particulate would accumulate in the substrate, pollutants such as nitrate and phosphate would increase to toxic levels, and the pH and alkalinity of the system would drop. Essential minor and trace elements would be depleted as well. Water changes remove these and other pollutants while at the same time replenishing diminished elements.
What's the best way to do a water change?
Using a gravel cleaning siphon. These easy to use devices not only remove water from the system, but at the same time will remove particulate from the gravel bed. These particulate will, over time, decay and degrade water quality. Gravel cleaning will also keep the gravel "open", allowing for a free exchange of food and oxygen between the water and the bacteria living on the gravel. Simply removing water from the system, while removing dissolved pollutants, does nothing to remove particulate in the gravel and is not as effective a method as using a gravel cleaner.

How much water can be changed at one time? Virtually all of it, should an emergency arise. Under normal conditions, you should not have to remove more than 10% to 25% of the tanks' volume. And don't worry about doing water changes while the system is cycling...it will not affect the cycle at all, after the initial 2-3 weeks.
How often should water changes be done? That depends on water quality parameters and bioload. A chronically overstocked tank will require larger water changes more frequently than will an understocked tank. Normally, 10% once a week or 25% once a month is adequate. If you notice that water quality is not improving, you can increase the amount, frequency, or both. However, try not to exceed a 50% water change if it can be helped. As an example we recommend a 5 gallon change on a 55g tank preferably weekly.
Water that is being added back to the system should be at the same temperature as the water in the tank, ± 2ºF, and free of chlorine, chloramine, heavy metals, and other pollutants. Using a "slimy" water conditioner or a chloramine remover will usually help ensure that incoming water is satisfactory. In some agricultural or industrial areas, incoming water should also be checked for nitrate and phosphate. If significant quantities of either are found, further treatment may be required to remove these pollutants, or the hobbyist should consider using reverse osmosis (RO) or deionized (DI) water.
Also at this time you should add all your supplements such as salt, pH stabilizers, buffers, etc. to this water.

Water Changing Tips:
Use a gravel cleaning siphon whenever you do a water change. This way, you will remove trapped particulate from the gravel at the same time you are removing dissolved pollutants.

While the use of auto-filling siphons is convenient, adding raw untreated tapetater into your tank can lead to disaster. Always precondition water before adding it to your system.

Never use hot water to adjust temperature. Heavy metals such as copper and lead can be found in greater quantity in hot water. In addition, some older water heaters may leach toxic amounts of zinc into the water. Heavy metals can be removed from tapetater by the addition of "slimy" water conditioners. These products contain chemicals that will bind these metals, rendering them harmless. Carbon and certain resins will also remove heavy metals from water.

More frequent, smaller water changes are usually more desirable than one large water change. Doing this will keep the water quality from changing to drastically.

Don't worry about water changing during cycling. Normal water changes will not affect this process, after 2-3 weeks.

For reef systems, or in areas where tap water is not up to par, the use of reverse osmosis (RO) or RO deionized water is recommended.

Thoroughly check well water before using it. Many wells are now contaminated with agricultural and industrial pollutants.

Always allow your tap water to run at least five minutes before using it. This will aid in decreasing the dissolved heavy metal content.