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Aquarium Basics: Partial Water Changes
The secret of a long-lasting, trouble-free aquarium is to do the regular maintenance right!

 Age of Aquariums > Aquarium Articles


One of the most frequent and most fatal mistakes made by beginners is to think that they need to "clean-up" the tank every now and then. By this they mean: scoop up all the fish into a bucket, remove everything from the tank, give it all a good scrubbing, assemble everything back, and dump the fish back in. Poor fish! This is completely wrong under normal circumstances, and hopefully this article will help you understand why.

Good, consistent water quality is the single most important element of a healthy aquarium. To maintain water quality, a regular schedule of partial water changes is essential in most aquarium setups. The reasons why partial water changes are so important (as opposed to total "clean-ups"), involve the concepts of Basic Aquarium Water Parameters and The Nitrogen Cycle, which you should be familiar with in order to take best advantage of what follows here.

The first part of this article will describe one of many good ways to do a simple 25% water change for a 40 L (10 G) aquarium. It assumes you have a gravel substrate, artificial decorations, external power filter, and few or no live plants - a typical beginner’s aquarium. Part two suggests some ways to decide how much and how often you need to do your water changes for your particular situation.

Part One: How to do a Partial Water Change

You will need:

From the housewares store: From the aquarium supplies store:
  • 2 plastic buckets, 10 liters (2.5 G) each
  • 1 plastic 2 liter (2-quart) pitcher
  • 1 set of plastic measuring spoons
  • 1 plastic turkey baster
  • Paper towels
  • 1 wooden chopstick (optional but useful)
  • 1 plastic scraper
  • 1 gravel siphon
  • Water conditioner
  • 1 algae scrubber
  • filter replacement cartridges or media
  • Keep your water-changing supplies and equipment together and reserve for aquarium use only. If your tools are used for other purposes such as household cleaning, residues from the chemicals used for cleaning may harm your fish.


    1. Fill one of the buckets with fresh tap water. Using the measuring spoon, add water conditioner to the water and set aside on a counter top.

    2. Unplug all electrical units: filter, heater, lights, air pump.

    3. Clean the inside glass of the aquarium using a folded paper towel, algae scrubber, or plastic scraper, depending on how tough your algae is. To get into tight corners, wrap the end of the chopstick with paper towel and use that. Do not use sharp or scratchy objects on an acrylic tank.

    4. Place the large end of the gravel siphon into the tank. If not self-starting, use the turkey baster to start the siphon by squeezing the bulb, inserting the narrow end into the hose end of the siphon, then releasing the bulb and pulling it out of the hose to pull the water through. If you have trouble with this at first, ask for someone's help. Sometimes you need an extra pair of hands, but eventually when you get the hang of it, you can do it by yourself. Have some old towels ready in case you spill some water.

    5. Once the siphon is started, vacuum the gravel. Push the tube straight down into the gravel and then pull straight up. Gravel will fall back down and debris (or ‘mulm’) will be pulled up with the water. Repeat as you move across the bottom. Do not sweep the tube through gravel as this will stir up debris and cloud the tank. You will not be able to completely clean the gravel when you do a water change. Do a different section of the gravel each time you do a water change.

    Tip: The deeper your gravel is, the more mulm will be trapped in it and the harder it will be to clean. This can contribute to water quality problems over time. With a basic, unplanted aquarium with no undergravel filter it is helpful to have a shallow layer of gravel, just enough to give the look of a gravel substrate - about 3 cm deep. Suction cups can be used to replace the plastic base of plastic plants to secure plants to the glass bottom of the aquarium under the gravel so that a deep layer of gravel is not necessary to hold down plastic plants. These suction cups are available in hardware stores or aquarium supply shops. Plastic plants can also be attached to rocks by removing the plastic base and ‘gluing’ the plant onto the rock using aquarium silicone.

    6. Fill the bucket with water from your aquarium, taking care not to overflow the bucket. Again, having someone help you with this the first few times will help you get the hang of it without major spills.

    7. Refill tank with clean water: Carefully tip the bucket to pour fresh water into the plastic pitcher, and then slowly pour water from the pitcher into your tank, being careful not to pour so fast that it disturbs the gravel. You can break the flow of water by putting your hand in the tank and pouring the water into your hand. Repeat until tank is refilled.

    Tip: If your new water is cooler than the tank water, take care that it doesn't suddenly chill the fish. Watch your thermometer and make sure that your water change doesn't cause a significant drop in water temperature. Adding water very slowly or in a couple of stages can help. If your water is very cold, allow it to stand longer and come to room temperature before adding it to your tank. You may also fill your bucket the night before the water change. Or adjust the temperature at the faucet to approximate the aquarium temperature if possible.

    8. Follow manufacturer’s recommendations for changing filter media (cartridge, sponge, floss, etc...) according to the type of filter you have. Dry your hands, wipe cords and plugs with dry paper towel and plug in all your electrical units again and check for proper operation. Maintain drip loops on all cords used for your aquarium to prevent electrocution and fire danger. To make a drip loop, make sure that every cord hangs straight down below your outlet and forms a 'U' before running back up to the plug. Any water that might get on the cord will drip down at the bottom of the loop and will not climb up to the outlet.

    Tip: Many filter cartridges can be reused many times. Carbon will lose effectiveness, but it only lasts a short time anyway. Try rinsing the used cartridge well with a strong spray of water if possible. If it comes out pretty clean and is still in good condition, you can reuse it. Shake out excess water, then set it aside to dry. Keep a dry one ready to replace in the filter as if it is "new". Reusing cartridges will save you money. Throw it away when it wears out and replace with new ones.

    9. Wipe down the hood and outside glass of your tank with paper towels. You may use a glass cleaner on the exterior glass, but take care that cleaning chemicals don't get into the water.

    10. Used tank water can be used to water plants, or discarded. Mulm is a good, natural fertilizer.

    11. Rinse out your equipment and allow to air dry, or dry with paper towels. Store in a dry place.

    Sometimes you will need to do a more extensive cleaning. The filter should be cleaned if there is a noticeable reduction in water flow. Keep in mind that when cleaning the inside of a filter and intake tubes of the brown, slimy buildup, you will be removing a large amount of biofilm, which is your system’s biological filter. If you clean all your tank’s surfaces at once, your nitrogen cycle may be disrupted which will be dangerous for your fish. Generally speaking, you can do a thorough cleaning of some other area along with your water change if needed. You can do a water change and a filter clean, or a water change and remove the plastic plants and decorations to clean them separately. If in doubt, collect some of the brown biofilm from inside the filter, keep it wet while you clean your tank, then put it back in your filter. It will not ‘re-dirty’ your filter but it will ensure that you have enough biofilm to maintain biological filtration in case you over-clean your system.

    It seems like a lot of steps, but once you get the hang of it, it doesn't take very long to complete this procedure from beginning to end.

    Part Two: How Do I Know When I Need to Change Water?

    Lots of factors can influence how fast your water quality declines, and there is no hard-and-fast rule about how much or how often you should change your water, because everyone’s system requirements are different. The only rule is to maintain your water quality so that it is as stable as possible in the artificial environment of the aquarium.

    To find out how much and how often you should change your water, test your water using test kits that are available at aquarium supply stores. Many aquarists use one of the following indicators to determine when to do a partial water change:


    In a soft water, low alkalinity (KH, buffering capacity) system, pH tends to decline. The metabolism of the nitrifying bacteria produces acids and these can reduce pH. For a beginning aquarist, if your pH tends to drop 0.2 or more over the course of a week, or if you are not able to maintain a consistent schedule for doing your water changes, it may be better to buffer your water (moderate KH) to reduce the necessity of very frequent water changes or the chances of a pH crash. Low KH water is desired for some fish, but most tropical community fish do not need it.

    The pH is more stable in higher KH water, so pH should not be used as a factor for determining the water change schedule in that case, because toxins will build up to high levels in the water long before pH begins to drop. If you wait too long and the pH drops, it will drop fast and may kill many fish. Test your water. If your KH (alkalinity or buffering capacity) is moderate or high, skip down to the next section and read how to use the Nitrate test to determine your water change schedule.

    When testing pH for the purpose of determining when to change water, try to test at about the same time each day since pH naturally varies slightly depending on the time of day. It naturally drops at night and rises again over the course of the day. This is normal and not of concern for your fish health or an indication that conditions are unstable.

    Your baseline pH is the measurement as it comes from the tap or after it has aged in an open container overnight. If there is a difference between those two measurements, use the measurement for the aged water. You may want to do your water changes with water that has aged overnight rather than freshly drawn water.

    If you notice a gradual pH decrease (less than 0.2 in one week) in your aquarium water from baseline, a regular weekly schedule should suffice to maintain reasonably stable conditions. It is less stressful for fish to do smaller, more frequent water changes (maintaining a stable pH) rather than larger, infrequent changes attempting to bring pH back up to baseline from low levels, because this causes the pH to swing up and down. Maintaining pH at or near your baseline value is important in case of an emergency and you have to do a very large water change. A good rule of thumb then, would be to do a water change whenever the pH drops 0.2 from baseline. After you become familiar with your water's behavior, you will be able to determine how often you need to change your water to keep it within 0.2 of the baseline pH. It may be weekly, biweekly, or monthly.


    Nitrate (NO3) is the indicator that many aquarists use to decide when they need to change water. NO3 will gradually rise as a waste product of the nitrifying bacteria that detoxify ammonia and nitrite. NO3 is generally harmless at low levels (up to about 40 ppm), but as levels increase beyond, it can affect fish growth, health and vitality. Some fish are more sensitive to NO3 than others.

    Rising NO3 also corresponds with the increase of several other organic pollutants in the water that are not measured by the aquarist, but which contribute to declining water quality.

    When nitrate tests reach the higher values of the "safe" range according to your test kit recommendations (or when they approach the highest level advised for your fish), it is time to change the water. Once you have a feel for how long this takes, you can schedule your water changes without having to test your water as often. You can anticipate when conditions are going to decline, and do the water change beforehand, to maintain more stable, good water conditions. You may need to change your water weekly, every other week, or monthly. Changes in fish population will increase or decrease NO3 production, so when you add or remove fish, you will want to test the water to see if any adjustments need to be made.

    If your system shows little fluctuation in pH and little rise in nitrate, you can schedule your water changes according to declines in general hardness or buffering capacity readings, or visible cues such as change in water color (slight yellowing) or the buildup of biofilm or algae on the glass which dulls your view of your fish. If you know about how long it takes before you see these indicators, you can schedule the change beforehand, so that your aquarium never looks like it needs maintenance. The idea is not to allow things to decline to a state where water quality becomes poor.

    It has been said, “Take care of your water, and your fish will take care of themselves.”

    Happy Fish keeping!

    Reader Comments Comment

    I recently purchased an Eheim Quick Vac. It's a tool that filters gravel in your tank without removing water. I find it very useful because you don't have to worry about moving a bucket when it fills up or dealing with a large amount of hose. Then when I want to change the water I can just set the hose in the tank and let it drain. Just thought I would add this. The tool does cost about 60 dollars though.

    Contributed by Victor

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