Disclosure: This post may contain text and/or image affiliate links where AquariumBlueprints.com can earn commissions if products are purchased.

How to change water in your aquarium without killing your fish

If your fish are dying after a water change, then there is definitely something wrong with your cleaning procedure. This guide will help you identify the mistakes you are making and will provide a step-by-step tutorial to changing your aquarium waters without killing your fish.

Quick Navigation:

Why your fish are dying after a water change

Not removing chlorine and chloramine

If you are adding tap water directly into your freshwater tank, chances are that you are introducing chlorine or chloramine. Both chemicals are designed to kill bacteria, which will make the water suitable for you to drink.

For your aquarium, however, the same chemicals are toxic for both your fish and the beneficial bacteria (which are needed to break down ammonia and nitrites). Thus, you need to remove chlorine and chloramine in your tap water before adding it into your fish tank.

The first method getting rid of remove chlorine and chloramine is by simply letting the tap water sit and age. Both chemicals will dissipate after several days. You can speed up this process by adding aeration via an air stone.

Boiling is a much faster way at getting rid of chorine and chloramine. It will only take a few minutes to get rid of chlorine and around an hour to remove chloramine. With that said, you still have to wait for the boiled water to cool down to the appropriate temperature for your fish until you can add the treated water into your tank.

The fastest way to remove chlorine and chloramine is with a water dechlorinator made specifically for aquarium fish. The best product, in our opinion, is Seachem Prime, which you can find on Amazon with this link. The solution will instantly neutralize both chemical; thus, you can add the tap water right away after dosing the appropriate amount.

Sudden and drastic shift in temperature

Another reason that your fish could be dying after a water change may be that due to a drastic, sudden shift in temperature. The swing will result in your fish possibly getting stressed or shocked, which may lead to illness and possibly death.

If you have an aquarium with tropical temperature, the water from your tap will most likely be cooler. As a result, you will lower the warmth of your tank by swapping out the old tank water.

The opposite can also happen. Say you decide to pre-heat your new water before adding it into your aquarium. If your new water is a lot hotter then the current temperature in your freshwater fish tank, then the sudden increase in temperature will cause stress and shock to your pet fish.

To prevent sudden and drastic shift in the temperature during water changes, we recommend swapping smaller volumes (25% at most) as opposed to bigger ones.

If you do want to do big water changes, we recommend pre-heating the new water with a heater so that your tank water won’t become too cold. If you don’t want your aquarium water to become too hot during a water change, you can let the new water cool off. You can also use an aquarium chiller if the new water is still too hot for your fish in your freshwater tank to handle.

When adding in the new water, even after adjusting the temperature, we suggest doing so slowly as an extra precaution. Just make sure you aren’t dumping in large amounts; instead, slowly splash in the new water to lessen the amount of stress on your fish.

Sudden and drastic shift in pH

Just like temperature, a sudden and drastic shift in pH will also harm the inhabitants in your fish tank. If there is a big difference in the pH of your tank and the source of your new water, then you might be putting your aquatic pets in danger during a water change. In general, we don’t recommend altering the pH by more than 0.2 (rather if you are trying to decrease or increase this value) over a 24 hour period.

Before anything else, you must first test the pH levels of both your tank water as well as your new water. If there is a difference of more than 0.2, then you should be careful when it comes to changing water.

One solution is to lessen amount of the volume you are planning to swap out. The only way to find out is by experimentation. Try swapping out a small amount of water and then test your water parameters shortly afterwards. If the pH stayed the same, then you can try bigger water changes.

Another option is to alter the pH of your new water before putting it into your fish tank. The fastest way to change pH is by using a commercial product, such as the API pH Up and API pH Down. Of course, you should test the parameters of your new water after dosing the solution before adding to your aquarium. You can purchase the pH altering products with the following links on Amazon:

Sudden and drastic shift in nitrate levels

Although the primary reason for a water change is to remove nitrates, removing too much of the compound can actually harm your fish. Just like pH and temperature, your fish will become accustomed to nitrate levels of your tank water. So a sudden and drastic shift in the concentration of nitrates can stress out your pets and even send them into shock.

The simple solution to this potentially deadly problem is to do smaller but more frequent water changes so that you can gradually lower the nitrates to a safer concentration over an extended period of time. Doing so will allow your fish to slowly but surely acclimate and adjust to the changing water parameters.

Killing beneficial bacteria

Another issue that could arrive during water changes is that you could be killing off your beneficial bacteria. You need these microorganism to convert ammonia into nitrites and then nitrites into nitrates. Even in small amounts, both ammonia and nitrites are deadly to your fish.

To prevent massive losses of beneficial bacteria, you first need to make sure that you remove the aforementioned chlorine and chloramine from your new water. As we stated previously, both chemicals are harmful to both your fish and beneficial bacteria.
You should also avoid removing the medias in your filter unless it is hindering the water flow. If you have activated carbon and/or filter floss, we recommend replacing both with more permanent filter medias such as bio balls, ceramic rings or sponges.

When you clean your filter media, it is best that you use the outgoing old tank water to rinse out all of the debris. As we mentioned numerous times already, tap water contains chlorine and chloramine, which will kill off your beneficial bacteria. You should also utilize used tank water to rinse off wastes from the decorations and ornaments.

Additional preventative measures

There are also a few additional preventative measures that you can take to prevent any fish deaths after doing a water change.

Live Plants

The first is to add live plants in your aquarium. These plants will consume ammonia and nitrates in addition to provide surface area for beneficial bacteria, which are going to remove ammonia and nitrites.

If you have never kept plants before, we recommend a beginner-friendly species called Java Moss, which is hard to kill and requires very little lighting. You can purchase a clump from Amazon with this link. Another good plant to control ammonia and nitrates is Pothos as you can dip its roots in your tank water. You can purchase Pothos on Amazon. To find out how to use the house plant for your fish tank, you can check out this guide.

Crushed Coral

We also recommend crushed coral in your tank to stabilize the pH if you prefer neutral (7.0) or slightly alkaline (above 7.0) waters. Your aquarium will become more acidic over time due the wastes produce by your fish. The crushed corals will dissolve if your waters become acidic, which will then slowly (and safely) raise and stabilize the pH.

For crushed coral, we highly recommend the CaribSea brand, which you can find on Amazon via this link.

Step-by-step safe water change guide

As you may have guessed from our previous statements, the safest way to do a water change without harming, or potentially killing your fish, is by doing smaller water changes. We recommend doing no more than 25% at a time. With that said, you can do bigger water changes if you can closely match the pH and temperature of the new incoming water with the tank water as well as have a low nitrate concentration (below 40 ppm).

If your new water is significantly different than the waters from your aquarium, you can follow our step-by-step guide for smaller water changes below:

1. The first step is to test both your tank water as well as the new water you plan on putting into your freshwater aquarium. You should test both for pH, ammonia, nitrites and nitrates. This will give you a sense to how much water you should be switching out. If you need a test kit, then we find that API Master Test Kit to be the best on the market. You can purchase a kit from Amazon.

If your tank is fully cycle, then you should have no traces of ammonia and nitrites (if you do see some spikes, you can check out this in-depth guide to combat this issue). So you should only worry about pH and nitrates.

For nitrates, you should be below 40 ppm. If you have more sensitive fish, or planning to raise baby fry, then you might want to consider going to below 20 ppm.

As for pH, we don’t recommend changing the pH of your aquarium by more than 0.2 over a 24-hour period.

2. The second step is determining how much water to swap out of your fish tank. We recommend that you base this off of pH. As we stated previously, you should experiment a little bit to see how much water you can swap out before altering the pH by 0.2 in either directions.

If the difference is small, then you can do a maximum of 25% (we don’t recommend doing more as the stability of the temperature and nitrate parameters are also important). If the difference is big, then you should consider doing smaller water changes of 10 percent of less to make sure that there aren’t any significant shifts in pH.

3. Now that you determine how much water you are swapping out, the third step is to unplug any electrical equipment you have in your aquarium as a precaution. For instance, a submersible heater may not be built to be exposed to air. If you lower the water line enough during a water change, then you risk breaking the heater. So, it is best to turn off the power of all equipment until you fill your tank back up again.

4. The fourth step is to remove water from your fish tank. You can do so by scooping out the tank water via a clean cup, bowl or container. Alternatively, you can use a hose or a dedicated water changing product like the Python No Spill Clean and Fill Aquarium Maintenance System, which you can purchase on Amazon with this link.

If you are planning to clean decorations, ornaments and/or filter medias, you should first put the outgoing water into a bucket. You can then rinse the wastes off of the decor, ornaments and/or filter medias in the bucket.

A bucket is also helpful if you keep small fish and other aquatic animals. This is because that there’s a chance that you may accidentally remove your pets during a water change. By putting the outgoing water in a bucket first, you have the opportunity to save them as opposed to sending the tank water directly to the drain.

Keep in mind that you are lowering the nitrate levels by cleaning off the visible wastes from your fish tank. As we stated above, you can lower the nitrate concentration by too much, which may lead to stressed and possibly dead fish. So we don’t recommend cleaning everything at once. Instead, you should rinse off only an object or two during ever water change.

5. Now that you are done removing the old tank water out, the fifth step is to use a water conditioner that will get rid of chlorine and chloramine from the new water. You can simply dose to what your freshwater aquarium is capable of holding directly into the tank (i.e. if you have a 55 gallon, put in the amount of the conditioner required for the entire 55 gallons).

6. The sixth step is to add the new water into your tank. We recommend doing so gently and slowly to avoid creating sudden currents that could stress out your fish.

7. Afterwards, the seventh and final step is to reconnect and power on all the equipment you have in your aquarium.

If your nitrate levels is still high (which is above 40 ppm in most cases), we recommend waiting 24 hours and then do another small water change. Continue to do small daily water changes until you can get your nitrate concentration under control.

Share this: