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How to properly do a water change to your fish tank

To keep your pet fish from getting sick, you will need to do a water change periodically.

This guide at Aquarium Blueprints will help you determine how frequently and how much volume you should swap out water in your fish tank.

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Importance for aquarium water changes

In nature, body of waters get “refreshed” constantly, whether if it is by rainfall or through melting ice from the mountains. Being closed systems, aquariums can’t afford the same luxury as your fish have to live in the same small body of water. Thus, fish keepers have to perform water changes, in most cases, in order to keep the tank water clean.

Does your fish tank need a water change?

If you don’t change the water in your tank, you will have a buildup of organic wastes. Even if you have a fully cycled tank that instantly convert all the ammonia and nitrite, you most likely have to deal with increased nitrates as well as increased acidity.

Any traces of ammonia and nitrite are toxic to fish while a large concentration of nitrate (above 40 ppm for most species or above 20 ppm if you are keeping more sensitive fish) can be harmful as well. If your fish is sensitive to pH, then the lowered pH may also cause issues.

So, you will most likely need to perform regular water changes in order to maintain the water quality of your aquarium.

How frequently should you do a water change?

The frequencies of water changes depend on a few factors, the most important of which is the bioload of your fish tank. If your fish produce a lot of wastes, then you might want to consider doing weekly or even daily water changes. If your bioload is lighter, then you can consider biweekly or monthly water changes.

Another factor to consider is convenience. Daily water changes may not be the best if you don’t have the time to do them. This is especially true if you have bigger fish tanks as you have to spend more time moving around more gallons of water and cleaning the tank.

We recommend having a firm schedule. In our personal experiences, we found that that it is easier to forget doing water changes in a bi-weekly or monthly intervals. Weekly works the best for us as we can designate a day where we have some free time to perform maintenance. Not to mention most working people are used to weekly schedules anyway.

How much water should you change in your fish tank?

Assuming that your tank is fully cycled (which means that you are getting readings of 0 ppm for both ammonia and nitrite), the amount of water that needed to be changed depends on your nitrate readings. As we already mentioned above, most fish species can tolerate nitrate concentration of up to 40 ppm.

So, before doing any water change, we highly recommend testing your tank water. Sometimes you may need to switch out more volumes while other times you can get away with lesser amounts. You might not have to do a water change at all if your nitrate levels are far below the 40 ppm limit.

If you are looking for a test kit, we recommend the API Master Test Kit (you can read our review here and purchase the product on Amazon with this link). (#CommissionsEarned).

One important factor that you should consider is that your fish will have to acclimate to the new water parameters. Even though you are removing nitrate during a water change, you will most likely end up changing the temperature and pH as well. Therefore, we recommend doing a water change of 50% at most in your tanks. If you have sensitive fish that can only tolerate a small range when it comes to water parameters, then you might want to switch out even less amounts during the water changes.

Of course, you can change out more water if you pre-treat your new water to match the water parameters in your fish tank. You can do this by putting a heater or chiller in whatever container you are using to hold your new water to match the temperature in your tank. There are also additives to lower or raise the pH. With that said, these steps will prolonged the water change process and make it more complicated than it needed to be.

Therefore, we recommend being more cautious and do water changes of 50% at most. If doing so doesn’t bring down your nitrate levels to a safe zone, then you can do another water change around 24 hours later. You should keep doing water changes, with around a day between, until you get your nitrate level down.

If you want to reduce the amount of water changes, in terms of frequency and quantity, you can check out this guide.

Test your water source and tank water before performing a water change

A water test kit is vitally important when it comes to performing water changes. With it, you can determine how many gallons of liquid you need to switch out.

While this is entirely optional, we recommend testing your water source first, rather if you are getting new water from your tap, water well or other places. If the incoming water contain a little bit of nitrate, then you might want to consider more switching out more water than usual to lower the nitrates in your fish tank to your desired level. If the pH in the incoming water is a lot different than the pH in your aquarium, then you might want to be more conservative when it comes to changing water. Sudden, drastic changes to pH and temperate will stress your fish and may even send them into shock.

The water parameters from your source can also change over time, rather if it is due to changes made to your water facilities, seasonal transitions and other factors. Thus, we recommend testing the source water a couple of times a year.

When it comes to testing the water in your fish tank, you should do it before doing a water change. Doing so will give you a guideline when it comes to what percent of water that need to be switched out. For example, if your API Master Test Kit has a reading of a Nitrate level somewhere around the middle of 40 ppm to 80 ppm, then you should do a 50% water change to bring the concentration to below 40 ppm.

We recommend testing your tank water every week, even if you aren’t planning on performing a water change. That way, you can keep a log of the nitrates in your tank.

Why you should use a water conditioner during water changes

Your tap water contains chlorine and chloramine, both of which will kill the beneficial bacteria in the fish tank. While chlorine and chloramine will eventually evaporate if the water is left untreated, this process takes several day.  Even if you are using well water, the liquid could contain potentially fish-toxic heavy metals, which a water conditioner will also remove.

A water conditioner we highly recommend getting for water changes is the Seachem Prime. Not only will it remove chlorine, chloramine and heavy metals, it will also detoxify the existing ammonia, nitrite and nitrate in your tank for up to 48 hours. You can check out our full review for more information. You can purchase Seachem Prime on Amazon with this link. (#CommissionsEarned).

How to dose your water conditioner during water changes

When dosing your water conditioner, you should do so before adding your new water into the tank. We suggest adding the dose directly into the tank and then add the new water.

For dosage amount, we recommend always dosing for the entire water volume of your tank. For example, if you have a 40 gallon fish tank, you should add enough conditioner for 40 gallons. Don’t worry if you overdosed a little bit. For instance, the Seachem Prime can be overdosed at up to 5 times the normal amount and will still be safe for fish in your aquarium.

Do you need to clean your filter during a water change?

A lot of fish keepers like to also clean their filters during a water change. There is usually a buildup of decomposing fish food and fish poop that accumulate on your filter media. As long as your water parameters are safe (i.e. you have 0 ppm of ammonia and nitrites in addition to less than 40 ppm of nitrates), than you don’t have to worry about cleaning your filter.

If you are having a hard time keeping the ammonia, nitrites and nitrates in check, even after doing a sizeable water change, then you should definitely clean out your filter. If you also notice a reduction in water flow, that is also a good indicator to perform some maintenance.

How to clean your aquarium filter

The most important part of cleaning your filter is to rinse off the detritus using used tank water that you remove from your aquarium. This is because tap water, when not treated with a water conditioner made for aquariums, contain the beneficial bacteria killing chlorine and chloramine.

Unless your filter medias are falling apart, we don’t recommend replacing them. This is because these provide a lot of surface area for your beneficial bacteria. If you replace your filter medias, you will kill off a good portion of your colony, which will throw your tank off balance in terms of being able to handle ammonia and nitrites. As a result, your tank will need to be cycled again, which puts your fish and other aquatic life in danger.

The only filter medias you should consider replacing are the products that are designed to be temporary such as activated carbon and filter floss.

Do you need to clean your substrate during a water change?

Similar to filters, your substrate will also collect decomposing fish food and fish poop. As we stated before, as long as your water test kits shows that the parameters in your aquarium are safe, you don’t have to worry about cleaning your substrate. If you are having trouble keeping nitrates in check after water changes, then you should remove the buildup from the bottom of your tank.

How to clean your substrate during water changes

Cleaning your substrate can be tricky. One method that you can try is by using a fish net to scoop up the detritus. If some of the buildup is stuck, you can try creating a small whirlwind with your fish net and then collect the wastes while they are waterborne.

You can also use tubing, such as the Python No Spill Clean and Fill Aquarium Maintenance System , to vacuum the substrate. The detritus will most likely weigh less than your substrate. By controlling the water flow of your tubing (either by pinching the tube or using a check valve) you will be able to siphon out the buildup while keeping you substrate intact. You can find the handy Python No Spill Clean and Fill Aquarium Maintenance System on Amazon. (#CommissionsEarned).

How to recycle used tank water

Although you can pour your used tank water straight into the drain, you can also recycle it for other purposes. Our favorite method is to utilize the dirty but nutrients rich water to irrigate air purifying house plants.


Water changes are needed primarily to remove nitrates from your fish tank. To keep your fish healthy, it is recommend that you should have less than 40 ppm of nitrates.

In terms of how frequently you should do a water change, we recommend weekly as it is easier to remember and less troublesome if you are too busy during your work days.

Before doing a water change, you should test your water to see how much of the tank water needed to be switched out. We recommend doing a 50 percent change at most so ensure that your fish won’t be too stressed out by the sudden change to the parameters. If a 50 percent water change is not enough to lower your nitrates to safe levels, you can do another change a day later.

We highly recommend using a water conditioner before adding new water to your tank as the new water might contain chlorine and chloramine, which will kill your beneficial bacteria, and/or heavy metals that can harm your fish. Seachem Prime will also detoxify ammonia, nitrite and nitrate at up to 48 hours per dose.

You don’t have to clean your filter and substrate during a water change if your water parameters are good. If switching out the water is not removing as much nitrates as you have anticipated, then you should clean out the detritus from your filter media and substrate.

When cleaning your filter, make sure to use the old tank water you are removing from the tank as using untreated tap water, which contains chlorine and chloramine, will harm your beneficial bacteria colonies.