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Why Small and Frequent Water Changes

(The First Tank Guide)

Why do small and frequent water changes, instead of big water changes just when the tank needs one?

OK, there are a couple of things that come into play here, percentage of water changed (which is different than volume of water changed - 1 gallon in a 1 gallon tank is 100%, 1 gallon in a 10 gallon tank is 10%, and 1 gallon in a 100 gallon tank is only 1%...) and frequency of water change (how often are you changing the water?), but what really matters, as many people will point out, is the difference in water chemistry and how stressful the water change will be to the fish.

Almost all freshwater fish live in water with constantly changing parameters - water chemistry, water clarity, luminosity, and temperature - so almost all fish will tolerate - or even thrive in - water within a rather broad range of conditions. However, very few fish are very tolerant of sudden changes in any of these, and even less tolerant of sudden and significant changes.

Now, over time, the chemistry in your fish tank changes - if nothing else, the nitrate level rises as the end product of your biological filtration. In addition, the overall impurity level will rise as water evaporates. This can impact a great number of parameters over time. Also, the natural biological processes occurring in a tank will tend to slowly push the pH down. All of these things are happening all the time in your tank. In addition, the water chemistry in your aquarium goes through changes in many parameters on a cyclical basis. In particular, this effects pH, nitrogen availability, and oxygen and carbon dioxide saturation. Also, over time fish (and to a greater extent live plants if you have them) strip many trace elements out of the water.

In addition, source water changes chemistry over time. If you are using tap water for your tank (which most people do), then the water will change as runoff levels change (if you have a surface water source) or as the water table rises and falls (for sub-surface water sources). Water parameters will also change as the water treatment facilities have to change how water is processed, based on microbe content, sediment levels, water temperature, and many other factors.

So, you have two changing sets of water parameters and, without a lot of testing (really much, much more than testing hardness, pH, and temperature), you have no way of knowing whether these conditions are converging (coming closer together) or diverging (going further apart).

Now, to return to where we started, the first potential problem with any water change is a change in water parameters that is both sudden and significant enough to measurably stress your fish, plants, or biological filter.

The easiest way to avoid this is by minimizing the opportunity for there to be a significant change in water parameters. So, how do you minimize the possibility of a significant change? Two ways.

First, make frequent water changes. This does not allow the water in the tank to drastically change, and minimizes the risk of significant changes in the chemistry of your source water. Second, make small water changes. If you perform small water changes, then the differences in water chemistry have to be very significant in order to make a noticeable difference in the water chemistry overall.

What are some benefits of frequent, small water changes?

If you frequently perform small water changes, then you never build up to a point where you have to perform big, messy, complicated, stressful, time-consuming water changes. Over time, frequent, small water changes will actually save you time, giving you less time fighting with your fish tank and working to keep it clean and healthy, and more time sitting back and enjoying your fish - or doing something else that you enjoy.

Another benefit of frequent water changes is that they do not have to be large to be effective. The longer you go between water changes, the larger the water changes need to be to effectively keep the tank healthy and stable. This large water change, in turn, poses a greater risk of stress to the fish, plants, and biological filter.

Frequent water changes are also easier on the fish because it is easier for them to get used to the routine. This reduces the stress of the water change in general because the fish sort of know what's coming.

Small, frequent water changes are also usually easier for the aquarium owner to schedule and get in the habit of doing. Thereby reducing the stress to the aquarium owner, which, in turn, can increase the enjoyment of the tank.

What are some drawbacks to large and/or infrequent water changes?

On the other hand, with a large water change, in addition to the risk of chemical differences or temperature differences in the new water, and the greater impact they can have on the inhabitants of the tank because a larger portion of the water was changed, the stress of a large water change is more significant on the fish, plants, and biological filter.

Even if water parameters are exactly the same, the process of either having the water level drop to the point where the fish feel that they are trapped in little divots in the gravel, or the stress of being chased around the tank to be put in a bucket, bowl, or spare fish tank for the duration of the water change is much more stressful to the fish than a brief, small water change.

Also, stopping the filter, which almost always has to be done in large water changes is stressful to the biological filter. In most cases, significant die-off in the bacteria bed can occur within the first few hours after the filter is disconnected, and this will result in the tank cycling, or at least partially cycling again once the filter is started again. This cycling process can again be stressful to the fish.

In addition, if you have live plants in the tank, the large water change is stressful for the live plants. Getting dried out and then re-submerged often results in damage to the leaves and some stems, which results in die-off, which, in turn, leads to an increase in decaying organic matter in the tank, and this can lead to algae problems, cloudiness, or nitrogen problems which can be very similar to those seen from 'new tank syndrome' when the tank is just cycling. Also, the act of catching the fish, or the activity of the tank owner, or just the weight of heavy, wet leaves and branches can cause damage to the plants which can lead to die-off or loss.

Any of these increases to the stress level of the fish can result in loss of color, elevated disease susceptibility, reduced ability to fight off a disease if infected, suppressed appetite, and hiding or cowering. In addition, the large water change itself can cause stress to the overall aquarium system which can dramatically increase the maintenance needed to keep the tank clean and healthy until it stabilizes again.

So, small, frequent water changes are better because they can keep the tank clean, minimize the stress on the tank and its inhabitants, and minimize the stress on the tank owner. While large water changes are not inherently disastrous, they do pose many significant risks to the tank and the tank population, which can all be easily avoided by switching to a schedule of smaller, more frequent water changes. I recommend a 10-15% water change weekly for most tanks.

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March 13, 2008
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