Let’s assume you have cleared out the organic wastes, did a water change, acclimated the fishes to the new water, and put them in the tank.

Everything is good and dandy until you wake up the next morning to see your fish unwell, or even worse, dead.

Water changes may appear simple enough; taking the fish out and reintroducing them after the change. But there are numerous mistakes people make while performing water changes. These errors can sometimes decide the fate of your fish.

In this article, we will discuss those mistakes, their outcomes, and the best solutions, so you can avoid a possible shock to your fishes during future water changes.

Why do we need to change the water?

When a fish lives in a tank, it becomes a part of its life. But, unlike our respiratory system, the water cannot circulate gasses and toxins on its own. It is a closed system, so we must ensure it stays clean and healthy for the fish.

Filters cannot weed out entirely toxic substances that have accumulated over time. Thus, water change is the best way to get rid of those harmful compounds.

Elevated nitrate levels

Let’s assume you feed your fish as instructed; provide them food 2 to 3 times a day. The amount should just be enough that it is consumed in under 3-4 minutes.

However, there will be food particles left uneaten, which sink to the bottom and start to decompose gradually.

The bacteria living in harmony with the water will try to break them down into ammonia. As you know, ammonia is toxic to fish.

Moreover, it is further broken down into nitrites, which again break down into nitrates. Even though nitrates are not as harmful as ammonia, increased levels can send your fish into shock.

In-depth information on Aquarium Ecosystem is available here.


Besides removing the organic wastes and nitrate, water changes also bring new oxygenated water to your aquarium. But, If you foster a lot in your aquarium, the oxygen levels will drop pretty quickly, despite of air pumps.

Control of diseases

If one of your fishes is diseased, and you transfer them to a different tank, some parasites might remain in the water.

These parasites are minute and sometimes the filter won’t be enough to get rid of them in water. In this instance, a complete water change will be your savior.

Also, if you put antibiotics or any other kind of medication to treat your fish, the chemicals will still linger around.

And, if the concentration gets too high, these medications may prove to be more harmful than helpful. So, a water change might be the only option to get rid of them.

Read more on Common diseases on Aquarium Fish here.

How to change the water for your tank?

Now that we have discussed the importance of a water change, let us go over 4 easy steps on how to carry it out-

  1. The first step is to clear out the detritus and organic waste accumulated over time in the substrate. Vacuuming is the best way to achieve this.
  2. After that, you can use a simple pump to slowly get the water out. And, if you don’t have one, you can use a mug and a bucket to remove the water. This process can take some time, depending on the capacity of the tank.
  3. Once the water is pulled out, and the substrate cleaned up, fill up the tank with dechlorinated water. You can also use the water from your filter to fill it up.
  4. After this, let the water rest for a day or two in the tank, for the bad gasses like chlorine to dissipate from it. Plus, letting the water rest will also bring it to room the temperature. After this, you can put your pets in the tank, but make sure to acclimate them before doing so.
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Pro Tip- It is better not to clean your filter during a water change. It contains useful microorganisms that can help you get rid of ammonia. Rather, clean the filter after a few days, so the microorganisms can properly dissipate from it and adjust themselves to the new water.

What can go wrong after a water change?

Many new aquarists may believe that a water change is much better than keeping the old one. Technically, it is true, but the answer is much more complicated than that. Sometimes it can prove fatal to the fish, especially if the water is drastically different than before. But why is it bad for the fishes?

New Environment

Fishes tend to adjust themselves with time in an aquarium as the changes in nitrogen concentration, and other parameters are slow.

But a sudden change in the water brings a drastic shift to their adapted body, and they fail to adjust to it in time. If the change is too much, the fish can get highly stressed, and go into shock. In worst-case scenarios, they may even die soon after.

Low water temperature

Fishes cannot regulate their bodies’ temperatures on their own. Thus we need to maintain it for them. If you do not let the newly changed water rest before putting your fishes in, it can lead to a thermal shock.

Some fishes may be too sensitive to the temperature. They may not be able to tolerate the instant change, and might die. However, if the fish manages to survive, it becomes more susceptible to diseases.

Too much Chlorine in water

If you forget to use a chemical to dechlorinate the tap water after the change, it can be fatal to your fish. Tap water contains chlorine, and sometimes combined chloramine, which is used for disinfection, is deadly for small aquatic organisms, even in low concentrations.

The substrate/filter was excessively cleaned

Used substrate and filters contain beneficial microorganisms that help keep the ammonia/nitrogen levels under control. It is good to clean them up, but excessive cleaning can kill all the bacteria present in them.

You should provide your tank some time after a water change so that the bacteria can multiply and attain a balance with the new environment. If their numbers are low, they won’t be able to break down the ammonia properly, which can prove life-threatening to your fish.

Saturated gasses in water

When there are too many gasses dissolved in the newly changed water, they try to get out in the form of bubbles. This happens because the liquid tries to attain a balance between itself and the pressure outside (barometric).

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The pressure changes caused by this condition can lead to the formation of gas bubbles in a fish’s blood vessels or even eyes. This will lead to permanent impairment of vision, and in many cases, the fish can die a painful death.

Disruption of osmoregulation

Aquarium water contains numerous ions that help in the regulation of water. A water change will cast them aside, which can result in disruption of proper osmoregulation.

This may or may not prove to be dangerous, depending on the fish’s biology. Some fishes are considered hardy and can tolerate changes, while some cannot and fall sick quickly.

How to know if your fish is in shock?

As discussed above, water changes can have bad consequences for your fish. However, there are a few signals or behaviors that you should look out for, after a successful water change.

Fish acting weird

If you see your fish moving in a strange manner, it might be in a lot of strain. This can be caused by sudden changes in water parameters, or an increase in ammonia concentration.

Your fish may be swimming in an abnormal fashion than it used to. This is a clear indication that the fish is unwell and under some stress. The possible reasons could be the temperature change, change in the hardness, or increase in ammonia in the water.

Ammonia levels can elevate in new water as the beneficial microorganisms are also adjusting themselves to the new surroundings. So, they fail to properly break down the ammonia released after the food starts decomposing.

Fish laying on the bottom of the tank

This behavior may indicate that your fish is lacking the energy to move. The bodily tension after a water change can make the fish feel lethargic, and it may prefer staying at the bed of the tank. This is bad news, especially when your fish is not a bottom-dwelling variety.

However, this can also mean that your fish is very sick and needs immediate treatment. Plus, elevated nitrogen levels can also make your fish less active than usual, and they might prefer to stay idle rather than swim. So, it’s better to be extra careful after a water change.

Fish gasping for air

In a newly filled tank, the water may not contain as much dissolved oxygen as required, especially if you own a lot of fishes. This can lead to your fish gasping for air at the surface.

In addition to that, if the temperature of the water is significantly warmer than usual, the dissolved oxygen concentration will be lower comparatively. This can lead to fish going to the surface to get air through their mouths.

And, there’s another reason behind this. As mentioned above, ammonia levels can surge very quickly after a water change. This, in turn, can lead to damage to gill cells, disrupting efficient oxygen exchange. This can force the fish to up and gulp air through their mouth.

Fish swimming upside down

If you are familiar with the term “dropsy”, then you may have already figured out why the fish are swimming upside down. It may sound weird, but newly changed water can sometimes lead to dropsy.

Yes, we are talking about the same dropsy that makes fish bloat. As discussed above, a fish has the ability to regulate the exchange of water and ions through its skin and gills, a process called osmoregulation.

Used tank water may contain a lot of ions that help keep the balance of ions between the fish and water. But the new water lacks those ions, resulting in too much absorption of water by the fish, leading to osmotic shock.

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This osmotic shock can lead to swelling of organs and impairment of the swimming bladder, resulting in fish swimming upside down. This phenomenon is commonly seen in Betta fish after a water change.

How to save a dying fish after a water change?

If you see your fish acting in any of the ways mentioned above, you need to immediately take action to sort it out.

Maintain the temperature

First, you need to check the temperature of the water. A thermometer will do the job quite well. Now, if you want to cool it down, use a fan just above the surface. A room air conditioner will also work.

To warm it up, a heater can be a perfect option. However, remember that you are trying to raise the temperature slowly, so that the fish doesn’t go into thermal shock. Whatever you decide to use, make sure it is done slowly.

Keep the pH favorable

Each fish has its pH requirement, and too acidic or alkaline water will surely be fatal to them.

Besides, you can use a pH meter or pH strips to check the levels. Simply dip the pH strip into the water to see what color it shows.

Furthermore, you can use pieces of wild driftwood, or peat moss to gradually increase the pH. To make the water more basic, the best possible choices are dolomite beads and crushed coral substrate. These are the best natural ways to maintain the pH value.

If you wish, you can also use synthetic buffers. However, these are highly concentrated and require proper research before you decide to use them.

Dechlorinate the water

If you have used tap water to fill up your tank, you should always use a conditioner to convert the Chlorine & Chloramine into less harmful forms.

However, if you forget to do so in any way, your fish will not be able to survive for long. These compounds are potent and are known to kill living cells.

It can quickly damage the exposed gill cells, which in turn will make it harder for your fish to exchange gasses in the water. Gradually, it can cause suffocation to death.

If you are unsure about the presence of chlorine in your water, you can either check it using chlorine meters, or chlorine strips. A better option would be to change the water again.

Note- While using a water conditioner, always read the instructions on the amount required. Too much water conditioner can also result in your fish’s demise. These chemicals, in higher concentrations, can bind to oxygen molecules in the water, decreasing their availability for the fish.

Summary & Tips

These situations are common in the life of a fishkeeper. Besides, people learn from mistakes and years of experience. Even with all the precautions, there’s always a possibility to face these awful circumstances.

However, here are some simple tips & tricks you can adhere to minimize the risks-

  • Water changes should not be done suddenly. You should always follow a proper schedule.
  • Avoid cleaning the filter frequently. One a month or two should be enough.
  • Remember to use dechlorinated water, or use a dechlorination agent after, if you are unsure about the water quality.
  • After a water change, make sure to clean the filter only after 3 -4 days.
  • It is best to keep the fishes inside the tank during partial water replenishment or changes (25-35%).
  • The substrate contains beneficial microorganisms for your aquarium. So, avoid cleaning it thoroughly. Just remove the debris, and return it back to the tank.

About the Author

Victoria Lamb

Victoria is a freshwater aquatics specialist, fish keeper, and amphibian enthusiast. She has had more than 6 years of experience caring for aquariums and keeping several fish species, and her home boasts of 3 aquariums and a garden pond. Her goal is to educate fish owners on raising healthy and happy aquatic pets.

Career Highlights:

  • Has worked with several aquarium manufacturers as a consultant
  • Organized and hosted workshops on freshwater fish keeping at retail stores, educational facilities, and libraries
  • Released content for the amphibian community through her writings

Educational Highlights:

Bachelor of Science in Animal Behavior and Welfare

  • University of Lincoln, Lincoln, UK (2014-2018)

Writing Experience

Victoria has done ghostwriting for many aquarium and pet websites in the past. She has also worked for Canada's largest natural health magazine- ALIVE, with 300,000 monthly circulations as a freelancer. She had six published articles on animal behavior and welfare during her graduation for her thesis.

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