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Nothing is more important when it comes to aquarium maintenance than regular partial water changes. The adage “an ounce of prevention is worth a pound of cure” certainly applies here.

One frequently reads of products that will reduce the need for water changes. Chemical media (carbon and such) may adsorb, absorb or change certain pollutants, but there is a finite limit to this and in some cases the pollutants return to the aquarium water in a different form that is still harmful. Some forms of bacteria will also release these adsorbed pollutants back into the water. And such chemical media frequently also adsorb beneficial nutrients which further worsens the state of the aquarium. Water changes also replenish essential substances such as minerals and ions. In the final analysis, there is no amount of filtration with any media that can duplicate the benefit of a regular partial water change. Everything that goes into an aquarium remains there in some form until we remove it.

The water in the fish’s natural habitat is not static but constantly changing. There are water currents in streams and rivers; thermal currents in ponds and lakes; a regular influx of fresh water from rain and snow melt. Aside from all this, the ratio of fish to water volume is infinitely greater than what we maintain in any home aquarium. The toxins the fish expel—which include ammonia through respiration, solid excrement, urine, pheromones, and allomones—are regularly being dispersed. In nature, these substances are dealt with “naturally,” but the aquarium is a closed system and the operation of nature is greatly restricted. This “crud” is partially handled by filtration but no matter how much filtration you have on your aquarium, crud in some form continues to accumulate (Boruchowitz, 2009). The regular partial water change improves this situation considerably by injecting fresh water into the system as it removes a given quantity of the toxins. This is very important to the long-term health of the fish, and live plants will benefit too. The volume and frequency do somewhat depend upon the specifics of the aquarium, but as will be evident throughout this article, there can be no doubt whatsoever that the more water that is changed and the more frequently, the healthier will be the fish. And water stability is virtually impossible otherwise.

An aquarium without regular water changes is like living in a sealed room without fresh air. Anyone who has worked in an air-sealed office building knows about this; regardless of the air conditioning system, it just is not the same as opening the window for fresh air. Like that air, the water in the aquarium becomes stale very quickly. Fish are constantly taking in water via osmosis, and they remove the minerals and expel the waste water along with any pollutants removed by their kidneys; an average-sized tetra produces 1/3 of its body weight in urine every day, so this quickly accumulates.

Many aquarists wrongly assume that a rise in nitrates (or a drop in pH) is the trigger for a water change. But this is waiting too late; the health of the fish and the aquarium’s biology has already been compromised if it shows up in these tests. The whole idea behind regular partial water changes is to prevent this from occurring, by maintaining a truly stable biological system.

1. Removing pollutants

The natural bacterial processes that break down toxins in the aquarium do not remove them completely, but change them into another form that may be less but nonetheless still toxic; an example understood by most aquarists is the nitrification cycle whereby ammonia is changed into nitrite which is then changed into nitrate. More recent scientific studies have determined that nitrate levels above 20ppm are detrimental to most fish in our aquaria, and some will be negatively impacted even at 10ppm.

And there are many others. Waste can be processed but it remains—until the water is changed.

Pheromones and allomones. These are chemical substances released by fish, plants, and algae. Biology Online defines them thus:

Pheromones: chemical substances which, when secreted by an individual into the environment, cause specific reactions in other individuals, usually of the same species. The substances relate only to multicellular organisms. This includes kairomones [these are released by flowers].

Allomones are repellent pheromones that induce a behavioural or physiologic change in a member of another species that is of benefit to the producer.

Just as hormones affect the body they are in, so pheromones and allomones released into the environment affect the bodies of other fish. These vary in function: some are used as communication between fish of the same species in a shoal—sometimes as a warning of danger; some initiate spawning behaviour between mates; some limit and even prevent growth of both the fish that releases them and other fish in the tank; some are warnings of aggression to other fish; and some indicate the fish is under stress.

Reduction in microbial populations and their metabolites. Microbial population refers to all those microorganisms living in the aquarium water; included are pathogens—microorganisms such as a virus, bacterium, prion, or fungus, that causes disease. Maintaining fresher, more stable water by removing all these pollutants is one of the easiest ways to keep the fish healthy. Regular water changes can also help to keep algae from increasing.

2. Water Stability

This encompasses a number of factors. GH (general mineral hardness) and electrolytes must be replenished, and this requires either a water change or additives. Fish physiology will only function at its best under stable and appropriate levels, and these are important for the fish’s osmoregulation. Maintaining proper and stable pH and KH (carbonate hardness or Alkalinity) is also tied to regular water changes.

DOC (dissolved organic carbon) accumulates, and while this can be assimilated by plants, it is easy for DOC to overwhelm the aquarium, particularly one without plants, and this has serious consequences for the fish.

Nitrogenous substances in the water column such as ammonia and nitrite can be removed before they enter the nitrification cycle and thus help to keep nitrates lower. This is again more important in tanks without live plants. A rise in nitrates indicates a biological imbalance.

Total dissolved solids (TDS) refers to the content of all organic and inorganic substances that are present in the water in a molecular, ionized or micro-granular suspended state. These build up from many sources: water conditioners and any substance affecting water chemistry; fish foods; organic matter; ions, minerals, salts and metals. As TDS increase, the ability of the water to hold oxygen decreases proportionally. The level of TDS also affects fish directly [see my article on stress for more detail]. TDS can only be removed by water changes.

Replacing trace elements and minerals. These are important to the health of the fish, as well as the stability of the water chemistry. Over time they are used up or filtered out. If they aren't replaced, the pH of the water will drop. Furthermore, the lack of trace minerals will adversely affect the vigor and health of the fish and plants. Giving your fish fresh water regularly is akin to taking vitamins to remain strong and healthy.

[continued in next post]

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3. How to Do a Partial Water Change

Stating the obvious, a percentage of water in the aquarium must be siphoned out and then replaced with fresh water. The substrate may or may not need to be cleaned during the removal of the tank water. In planted tanks, it is best to leave most of the substrate untouched; in non-planted tanks cleaning of the substrate is usually recommended to remove waste and other organics that in the absence of live plants will likely increase nitrates. [For more on this, see my article on Bacteria in the Freshwater Aquarium.]

Devices made for water changes allow cleaning (or vacuuming) of the substrate without pulling the substrate material out. These work well with any gravel substrate; with sand substrates, it is best to lightly vacuum only the top so that the sand is not also sucked out.

Water changes in larger tanks are tedious using a water changer and a pail. The automatic “Python” apparatus that connects directly to a faucet is recommended.

Water conditioner should be added either directly to the water if using a pail, or to the aquarium as the water begins to enter if using a “Python” device. Sufficient conditioner for the volume of water being changed is adequate in either situation. If the aquarium holds especially sensitive fish, increasing the amount of conditioner may be advisable. Although most manufacturers say the product is not harmful if overdosed, this is misleading; all conditioners increase the TDS (total dissolved solids) and they are chemicals.

4. When not to do large water changes

As the bio-filtration cycle operates, whereby Nitrosomonas bacteria convert ammonia (produced by the fish continually) into nitrite and then Nitrospira bacteria convert the nitrite into less harmful nitrate, a by-product is the production of acids. Normally the buffering capacity of the water keeps this from becoming problematical, and the regular water changes are part of this safety net. Without water changes for several weeks, the acids accumulate; the larger the fish or the more there are, the more acids. We call this “old tank syndrome,” and as this is gradual, the fish adapt with it. There comes a point when the buffering capacity in the water is maxed out, and the pH starts to fall, and if it drops to the low 6's the good bacteria can't function and ammonia starts to build. In acidic water ammonia ionizes into less toxic ammonium which allows the fish to survive. Everything continues to look OK from the outside. Then you finally do a water change, which in this situation can be deadly.

The tap water pH will most probably be much higher than the pH of the tank water, and the more alkaline the tap water is, the worse will be the result. When half of the water in the tank is replaced with this fresh alkaline water, the pH in the tank rises and if it rises above pH 7 the ammonium in the tank water will change back into ammonia which is highly toxic, and the fish will succumb. The change in pH has a part to play as well, since the fish have to work extremely hard to readjust their internal pH to survive such a significant shift in the external pH, and this weakens them which only adds to the problem. It takes time for the Nitrosomonas bacteria to multiply to an adequate number to handle the sudden increase in ammonia, and similarly for the Nitrospira bacteria to handle the subsequent increase in nitrite. This mini-cycle is relatively short by comparison to a new tank, but it is just as stressful on the fish. In a thickly planted tank, where the pH is normally slightly acidic on purpose, the plants are able to take up much of the ammonium, which means the final result would not be quite as disastrous.


Boruchowitz, David E. (2009), “Time for a Change: A Mathematical Investigation of Water Changes,” Tropical Fish Hobbyist, November 2009 (Part 1) and December 2009 (Part 2).

Evans, Mark E. (2004), “The Ins and Outs of Osmosis,” Tropical Fish Hobbyist, February 2004.

Muha, Laura (2006), “Fish Growth vs. Tank Size” in ‘The Skeptical Fishkeeper’ column, Tropical Fish Hobbyist, December 2006.

Strohmeyer, Carl (2011), “Reasons for Water Changes,” American Aquarium Products website:

“Growth Inhibiting Substance(s) of Fishes,” on the Wet Web Media at

"Is Nitrate Toxic? A Study of Nitrate Toxicity" on the site.

Byron Hosking
November 14, 2012
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