Water changes are the most important aspect of aquarium care. Only diet ties with how important water changes are, and they are equally important. You can provide the best food or water quality, but if one is lacking it can undo the highest quality in the other.
There are many vital things that water changes do for the aquarium. A healthy aquarium change schedule can actually help save money when it comes to filtration since they can make the use of carbon filtration completely unnecessary. Nothing, absolutely nothing, can make up for a lack of water changes. Filtration can’t do it. Special additives and special types of filtration can’t do it, nothing can replace water changes.
It is important to understand exactly what filtration and water changes do so that you understand the difference. Filters do two main things. First is that they provide a place for the bacteria that consume ammonia and nitrite to grow. The surface are of biomedias and the flow of the filter create the perfect submerged habitat for these nitritfying bacteria. The other main function is to physically collect debris and particulates from the water column for the aquarist to remove during monthly filter maintenance. This is why it is vital to perform filter maintenance every single monthly, regardless of the type or brand of filter being used or whether the flow is reduced or not. Water changes maintain water quality long term. Long term referring to years and years, even decades.
The problem with water changes is that their results are long term. Many people only see short term. So if someone stops doing water changes or cuts back drastically (like only doing water changes once every four weeks) they may not see any major difference. The reason is that the problems that a lack of water changes cause short term are very subtle. Many people feel it is best to leave well enough alone and not fix what isn’t broken. This idea leads to neglect and the aquarium and all its inhabitants will suffer.
When inadequate or no water changes are done the tank may appear to do well. The fish will still be alert and swimming, they will still have colors, and they will still eat. This can go on for a few years. What I have seen happen under these circumstances is what I call old tank syndrome. This is when the water quality has gotten so bad that all of a sudden effectively every fish in the tank gets sick. I have seen this happen a few times, killing 90-95% of the fish. A sign that this situation may be occurring before it actually kills everything is when new fish are added. Usually a neglected tank’s chemistry will change over time. The pH will drop along with hardness and the water quality at this point is very low. Together these can prove fatal to new fish. The fish that experienced it slowly acclimated to the changing water as it slowly happened over time. New fish are acclimated in less than an hour. This sudden change can shock new fish, resulting in their death with no symptoms of any problems.
Water changes do two things simultaneously. Most importantly they remove all the bad things that build up in the aquarium over time. These things include nitrate, phosphate, growth inhibiting hormones that many if not most fish give off, dissolved organic compounds, and other things. Although nitrate is far from the only bad thing that builds up, it is the only only one we have test kits for and it does correlate very well with all of the other bad things that build up. This is why so many people make such a big issue out of nitrate. The only exceptions are heavily planted tanks where nitrate and phosphate may be removed, but not the other things, and when nitrate specific medias are used (such as Purigen). In these cases nitrate tests may show it is nice and low, but you have selected the one thing that you can test for and tricked yourself into thinking the water is better than it is. This is one reason why water changes are beneficial, even heavily planted tanks.
The other major function of water changes is to bring in good things that are used up over time. The end result of all the biological activity in an aquarium is acidification. This uses up the KH in the water. As the KH is used up the pH will decrease. Eventually the pH can crash. There are many other vital minerals and trace elements needed that are replenished with water changes.
Water changes have a massive impact on stocking. It is negligent to discuss stocking without addressing water changes and water quality. Discussing stocking based on tank size alone is no different than simply going by one of the very flawed ‘one inch per gallon’ type guides. Experiments have shown that it is the water quality, not tank volume, that stunts fish. Discus were raised in two different setups. One was a twenty gallon tank that was heavily stocked and received very large daily water changes. The other setup was a fifty five gallon tank with a small fraction of the number of fish and did not receive any water changes. The discus in the twenty gallon grew at a normal rate while those grown in the fifty five were severely stunted. The experiences of many other aquarists support the results of this experiment, demonstrating that it is definitely water quality, not volume, that is the active factor in stunting fish. Obviously there is still a minimum tank size for fish based on size, behavior, and activity level. Please see the Stocking an Aquarium article for more information.
How much, how often:
Water changes should be done on a weekly basis. This frequency gives little time for the chemistry in the aquarium to change significantly compared to the tap water. Over time the chemistry in an aquarium changes and allowing too much time between water changes allows the tank’s water chemistry to change too much. This means that a larger water change will change the tank’s chemistry too much and stress or even shock fish. The amount changed at each water change should be whatever is needed to keep the nitrate concentration under 20ppm. This means that if a weekly water change of 25% doesn’t keep the nitrate concentration under 20ppm then the amount of water changed every week needs to be increased. Any time the size of the water change is changed it should be done slowly. So you should not jump from a weekly 25% water change to 50%. Increase the amount by 5-10% each water change to allow the fish to acclimate to the new schedule and to allow you to catch any issues that may cause problems. Smaller water changes allow for more error than larger water changes. For example, if the temperature is not perfect for a 25% water change it may not cause any problems. The same temperature difference with a 50% water change could severely stress the fish. Another issue is that the water heater may provide enough hot water for a 25% water change but then run out shortly after.
Water changes can prevent and fix almost any problem:
Water changes are one of the most powerful ways to prevent and treat almost all problems that an aquarist is likely to encounter. A healthy water change schedule can prevent most of the problems an aquarist might otherwise experience. In my experience the health issues that arise in well maintained tanks are almost nothing compared to those that are neglected.
In addition, water changes alone are frequently all that is needed to fix most problems that may arise. Increasing the frequency of water changes when problems do arise may be the only action the aquarist needs to take when fish are stressed (lethargic, clamped fins, refusing to eat, etc.). It is also all that is needed in many cases of a variety of diseases. I have seen case after case of ich, bacterial infections, fungal infections, and others fixed by increasing the frequency of water changes alone, no medications used at all. It is also the main issue when algae problems arise. Please see the Algae Control article for more information. More or larger water changes are frequently the only change needed to fix algae problems.
How to do a 90% water change on a 300 gallon tank with only 10 minutes of hands on time: