Water Changes
by Shelie Wittig ( Fishgal )

One of the most important aspects of successful fish keeping is good aquarium maintenance, including routine water changes.


If your aquarium looks beautiful and the water is crystal clear, everything is wonderful… right? Unfortunately not.

  • In an established aquarium, bacteria convert ammonia (produced mainly by decaying food and fish waste) to nitrite, and nitrite to nitrate (NO3). Nitrate is the end result of the Nitrogen Cycle, and while it is the least toxic form of nitrogen it does build up over time – to potentially dangerous levels if left unchecked. Changing a portion of the aquarium water on a regular basis is the easiest way to reduce it.

  • The buildup of nitrates (nitric acid) can reduce the alkalinity of your water, potentially resulting in a pH drop.

  • Phosphates, pheromones and other chemicals can build up between water changes.

  • Minerals and trace elements are depleted over time, which can result in a GH drop.

  • Poor water quality can lead to infection on a wounded fish.

  • Poor water quality puts stress on fish, which is often the root cause of fungus and parasite problems.

  • It has been said that dirty water holds less oxygen, although I do not have any scientific data to back this statement up. Without question, water changes supply clean oxygenated water to your fish – and that’s a good thing!

How Much, How Often?

There is no standard answer to the question of how much water to exchange, or how often. It depends on several factors including the size of your tank, number of fish, feeding schedule and filtration system. Testing for nitrates is the most effective way to measure your water quality. As you begin to establish your maintenance schedule, perform a nitrate test before and after each water change, as well as once in between. As stated previously, NO3 builds up slowly and your goal is to keep it as low as possible; at the very least it should be less than 40ppm at all times. Once you determine a schedule that works for you, testing can be done less frequently to confirm your routine is adequate.

As a general guideline, you could start with a 20-30% water change once a week. Some people prefer to do a larger change less frequently, such as 40-50% every two weeks. I believe the latter is less desirable since the water chemistry changes more dramatically at once, putting more stress on the fish. Alternately, if your bio-load (ratio of fish to water volume) is low and you don’t overfeed, you may be able to do a smaller water change less frequently and still maintain good water quality.

Keep in mind as you determine the volume of water you intend to change, that a 100-gallon aquarium does not necessarily hold 100 gallons of water with substrate, rocks and equipment in the tank as well.


  1. If you wish to vacuum the gravel at the same time (a very good idea), remove any rocks or other decorations you want to vacuum under (be careful there aren’t any fish hiding in the nooks and crannies!) Many people opt to do this on a rotating schedule; Week-1 water change only, Week-2 water change and gravel vacuum, etc. If you have an undergravel filter system you will likely have to vacuum more frequently to keep nitrates at bay.

  2. If you have excessive algae growth on any decorations and wish to remove it, you can soak them in a weak solution of chlorine bleach and water (1 or 2 tablespoons bleach per gallon of water will suffice). Since some of your beneficial bacteria lives on the surface of these decorations it’s not advisable to do this too frequently, or at the same time you replace filter media. Also keep in mind that the green algae is highly nutritious and many fish (including Mbuna and Plecostomus) will feed on it.

  3. If you want to scrape algae off the glass this is a good time to do it. Use a non-scratching brush or sponge used only for your aquarium, and be certain it does not have any detergents or other cleaning agents imbedded in it by the manufacturer.

  4. Pre-measure your water conditioners (buffers, dechlorinator, ammonia neutralizer, etc.) for the amount of water you intend to change. You can do this in a bucket or other clean container used only for aquarium maintenance. (See "Water Treatment" for details on making tap water safe, and "Practical Water Chemistry" for information on buffering and other chemistry tips.)

  5. Unplug aquarium lights, filters and heaters. (Some people advise doing this before working in the tank to avoid the possibility of electrical shock if, for example, the heater were accidentally smashed. I prefer to at least leave the filters running to pick up some of the muck stirred up in Steps 1 thru 3.)

  6. Start the water siphoning. You can measure the volume removed by collecting it in a pre-marked container (large bucket or plastic trash can) OR by pre-measuring and marking an indicator on the tank to which you consistently bring the water level down. If you use the second method, you may still wish to collect some of the tank water in a container so that you can rinse filter media in it. (The chlorine in tap water as well as a fluctuation in temperature can destroy beneficial bacteria.)

    • For gravel substrate, a Python® type hose is ideal. Simply plunge it into the gravel and slowly pull it out. Debris is sucked up with the water, while the gravel falls back to the bottom. It is a common fallacy that vacuuming too thoroughly will remove beneficial bacteria. In truth, bacteria adhere to all the surfaces in your tank including the glass, substrate, rocks, plants and filter media. It is not in the muck you remove from the gravel.

    • For sand substrate, move your siphon hose or Python® in a circular motion just above the surface to pick up debris without sucking up the sand. You could also use a second tool (such as a PVC pipe or your other hand) to disturb the sand just before passing over it with your hose.

  7. Once you have removed the desired amount of water, replace your rocks and other decorations. (If you cleaned them in a bleach solution you must rinse thoroughly, until you can no longer detect the smell of bleach. After that, you can submerge them in fresh water with a dose of dechlorinator as an additional precaution.)

  8. Now you can begin refilling.

    • If you are using water straight from the tap, allow it to run for at least 5 minutes to reduce the concentration of copper and heavy metals from household plumbing. You can use this time to adjust the temperature as close as possible to that of your aquarium. A thermometer first placed in the tank then taken to the water source is very helpful. (There are claims that warm or hot water from your tap should not be used since it tends to have a higher concentration of copper and heavy metals. With the use of a product such as NovAqua® this becomes a moot point, and matching the temperature of your tank eliminates the possibility of shocking your fish with a temperature change. Personally, I have used this method for nearly 10 years without incident – as have many other aquarists.) Begin by adding water to the container in which you pre-measured the conditioners (if any). If this is a small receptacle or bucket, you can dump that into your tank after all the additives have dissolved completely, and add the remainder of the water directly from the tap to the tank.

    • Alternately, you can pre-fill a large container with water a day or two ahead of time and avoid using a dechlorinator, since chlorine naturally dissipates when exposed to open air. (You may still need to use a product to neutralize ammonia and chloramines. See "Water Treatment" for more information.) If your room temperature differs more than 1 degree from that of your aquarium, you should place a heater in the container as well. Obviously, you can also add your water conditioners to this container, then simply pour or siphon the entire contents into your aquarium.

  9. Restore power to your filters, heaters and lights.

  10. Record your activity in a maintenance log.

Dealing with High Nitrates

There are a number of factors that contribute to an ongoing nitrate problem in your aquarium. Consider the following:

  • The heavier the bio-load, the more aggressive your maintenance schedule will need to be.

  • The more you feed, the more nitrates will be produced (more food in… more waste out).

  • Some tap and well water is high in nitrates, negating the effects of water changes. If your water supply tests high for NO3, you may need to consider a reverse osmosis filter.

  • Undergravel filters can make nitrate control more challenging. Organic matter is drawn deep into the substrate where it is difficult to remove completely. If you struggle with high nitrates and keep African cichlids (many of which enjoy excavating), you might consider a different type of filter.

  • Live plants utilize nitrate like fertilizer and can help keep NO3 levels down.

  • Activated carbon adsorbs a minute amount of ammonia, nitrite and nitrate.

  • Products such as Nitrazorb® specifically target nitrates and are more effective than activated carbon.

Remember that nothing can take the place of routine water changes to keep your water clean and your fish healthy. □


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