How to Change Fish Tank Water Without Killing Fish | Be a Pro

Changing water is an essential part of aquarium maintenance. Many times fish keepers experience fish dying after a water change. But, is there a proven technique to do that? let’s see how to change fish tank water without killing fish in a systematic and step-by-step way.

Prepare the new water before adding it to the fish tank. Use a de-chlorinator to remove any chlorine present in the tap water, and preheat the water close to the tank temperature. After that, change only partial water at a time. Never combine the water change with tank cleaning or changing filter media.

You will just vacuum the substrate during the water change. A thorough cleaning of the tank, or changing the filter media on the same day may result in a significant loss of helpful bacteria.

Can A Water Change Kill My Fish?

Can water changes kill fish? Yes, water change can kill fish. Fish die after water change due to several reasons. They die mostly because of temperature shock, a drastic change in water chemistry, and the loss of beneficial bacteria.

Do you want to hear the real story of one of my fellow fishkeepers? He lost many of his fish overnights. The story can help you to avoid the same mistake.

The morning turns into mourning for him when he sees most of his beloved pet fish floating upside down. Everything was right after changing the water, he fed them one final time before going to bed, said goodbye, and went to sleep.

Do you wonder what went wrong? He performed a rigorous water change last day. So, was there something terrible associated with this action?

Did the water change kill his fish? Apparently, yes, but the actual answer is more explanatory than that.

He has been doing fine with a partial water change ever since he set up the tank over a year ago. One of his friends visited his house and noticed the substrate & the tank overall looked a bit dirty.

He suggested the aquarium owner for cleaning the substrate. The owner immediately took action, changed all the water, and cleaned the aquarium thoroughly.

Everything looked immaculate, and the owner was thrilled as the tank was looking stunning after the cleaning. But the next morning, half of the fish died, and the rest died within a week!

The owner was anguished to see all the dead fish and his friend blamed the tap water for being lethal. Was it really so? Let’s dig a little deeper.

Why did a water change kill fish?

The sudden alteration in water chemistry and aquarium temperature is the culprit for fish’s immediate death after water changes. Also, water changes can destroy the established nitrogen cycle in the aquarium resulting in added toxicity and stress. The further fatality comes from the second reason.   

Why did water change kill fish sometimes

Sudden change in the water chemistry

A drastic change in the water parameters imposes tremendous stress on fish. It is more damaging if you haven’t changed the aquarium water for a very long time and then replaced everything straight away.

An aquarium is a micro-ecosystem in which several biological and chemical processes are taking place continuously.

Fish wastes, food leftovers, and dead plant matter alter the water parameters over time. If this change is slow, fish can somehow be accustomed to it.

Though you are adding the purest water to the aquarium, the pH, hardness, and other water parameters shift from the previous condition.

Fish might not tolerate this change and die. Poor pets were attuned to some pollution, but the freshness is killing them!

Temperature shock

Another potential reason for fish getting killed by water changes is a sudden down in water temperature. When adding new water to the tank, it is vital to maintain the same temperature as the previous tank condition. 

If the degree of temperature difference is extensive, some fish may die going into shock, while others may succumb to the disease.

Losing beneficial bacteria

Inappropriate water changes are the slow killer of fish in another way. Over-frequent water changes can kill off beneficial bacteria in the fish tank. Beneficial bacteria are essential in establishing a nitrogen cycle that prevents toxicity build-up in a fish tank.

If you remove all the water and thoroughly clean the aquarium, it can destroy the bacteria colonies. A fish tank that is deficient in good bacteria requires cycling again.

Otherwise, toxic material will build up rapidly, and the water quality will deteriorate faster. The entire process becomes harsh on fish and their immune system weakens.

Pathogens living in the water might not miss this opportunity to kill your fish. If your routine water changes are too frequent to allow the growth of beneficial bacteria, the result will be the same. Therefore, knowing how often should you change water is crucial.

Why should you change the water then?

If changing water can potentially be fatal for fish, should you do it at all? The answer is, yes, you should make regular water changes for the long-term health and betterment of fish.

Dissolved toxicity in the aquarium water is not visible to the naked eye. It is like a slow poison, reduces fish immunity and, eventually, their lifetime, and can’t be eliminated unless fresh water has been added. 

Fish secrete waste, and it goes nowhere but accumulates at the bottom. If your fish don’t eat all the food, leftover food joins with fish waste at the tank bottom. 

In all feeding instructions, it is clearly briefed to feed fish as much as they can finish in 2-3 minutes. That is a very vital instruction as fish foods like flakes or pellets don’t float for hours. If your fish don’t consume floating foods right away, they will sink to the bottom.

All the aquarium wastes start breaking down at a point and decomposing to ammonia. Ammonia is a highly toxic gas that remains dissolved in the water unless bacteria convert it to nitrates.

The entire biochemical process is known as the nitrogen cycle. Establishing a nitrogen cycle is essential to an aquarium’s health. 

The toxicity of ammonia is acute and fatal. Nitrates are less deadlier than ammonia, not killing the fish outright, but still harmful, and need to be in check.

Live aquarium plants can consume nitrates naturally. Therefore, aquariums containing plenty of live plants may not require a water change altogether.

However, sometimes it may not be sufficient. One of the most effective ways to remove nitrate from the tank is by changing the water.

Another crucial reason for regular water changes is that it can oxygenate the aquarium water. An oxygen-rich aquarium is a healthy environment for fish.

The aquarium water condition impacts the fish’s immunity system directly. A healthy fish with strong immunity will rarely get sick.

How to change fish tank water without killing fish

Acclimatization is the key! Don’t impose your fish on a sudden change. Despite a small risk that it might harm your fish, regular water changes are incredibly crucial. 

You cannot compromise by subjecting your fish to poor water quality, and you cannot risk their life. So, you have to know how to perform water changes in a balanced way. Be mindful of four basic things.

  • Do only a partial water change 
  • Use a gravel vacuum
  • Be cautious about water chemistry
  • Maintain a stable water temperature
How to Change Fish Tank Water Without Killing Fish tips

Step 1: Do only partial water changes

Small water changes regularly are more effective than a massive water change at a time. I suggest changing only 10-25% of the water every week. To eliminate any excess nitrate buildup, it is safe to perform a one-third water change monthly.

I’ve found it’s a perfect routine with no sudden change in water condition and not losing friendly bacteria. Partial water change this way is a great prevention of nitrate buildup without disturbing your fish.

Step 2: Use a gravel vacuum

People make a mistake changing all the water when they go through a thorough tank cleaning, including the substrate. They find no better alternative to clean the substrate without removing tank water. 

It is advisable to use a gravel vacuum to suck out debris from the bottom once a month while performing the major cleaning.

A gravel vacuum is a useful tool to perform water changes and substrate cleaning. So invest in a better quality tool. Siphoning helps water change and clean a fish tank conveniently.

Step 3: Care about the water chemistry

Water chemistry is crucial for the fish’s health, and an extreme change can be deadly. Therefore, maintaining a steady condition before and after the water change should be the top priority. 

Before you add new water to the aquarium, perform a simple water test to analyze pH with a testing kit. Though several parameters control the water quality, pH is one of the key indicators

If you add tap water, don’t forget to treat it with a conditioner. Effective water conditioners are readily available in the market. The aquarium water needs to be free from chlorine and heavy metals.

Step 4: Maintain a steady temperature

Always try to keep the new water temperature as close as possible to the current aquarium water temperature. A quick change in temperature can be severely stressful for fish

For most tropical fish, the acceptable temperature range is 75°~82° Fahrenheit. Fish are more susceptible to diseases at temperatures lower than the ideal range.

You can easily measure the water temperature with a thermometer. It is better to preheat water before adding it if necessary. 

Water change sequential steps

Fish-tank-water-change-sequential-step by step

Arrange tools

The primary tools for water-changing tasks are a siphon and a bucket. A siphon-type gravel vacuum is handy for this task. Additionally, a water conditioner and a preheater are necessary to prepare the refill water.

Prepare new water

The preparation of new water to refill the aquarium is a crucial step. Tap water is perfectly okay if it is chlorine-free because chlorine compounds are deadly for fish.

It is better to use a water conditioner or de-chlorinator. When using a water conditioner, don’t forget to choose a reliable brand, and follow the manufacturer’s instructions to the letter. I recommend Seachem Prime (usable for both fresh & saltwater aquariums).

Once the water is ready, give a final inspection. Check the pH and water temperature. If the water is too cold, preheat before adding.

Siphon out water

Place the suction of the siphon just above the substrate layer and suck water from the bottom. I recommend a 10-25% water change every week, and not more than 30-40% if you do that monthly. Keep in mind that changing more than 50% of water at a time is not wise.

Add water to the tank

The final step is pretty simple. You have prepared a bucket full of water to add to the aquarium. You have tested the pH, and the water temperature is just perfect as you desire. Now add it to the aquarium, and your job is done.

Do you leave fish in the tank when changing the water?

It’s absolutely fine to keep the fish in the tank while changing water. In fact, there’s no point in removing the fish to another place.

Transferring the fish to another tank may cause stress to your fish. Moreover, you’re not going to remove all the water during a water change.

How to do a partial water change without a siphon?

A partial water change can be done without a siphon in three different ways. Using a tool dedicated to changing water where no manual siphoning is needed is the best way. In addition, a canister filter can be utilized to drain and refill the tank, or for a small tank simply using a small water pump or bucket is good enough.

Change Water Using Tools:

The most effective tool I found for water changing and vacuuming the gravel is the Python water changer hose. It’s a real game changer for fish keepers.

Python water changer
Python Water Changer

It is very much affordable, and it offers a hassle-free water drain and fill-up. You can connect the Pyhton tube directly to the faucet, and it works both ways. You can drain or refill the water through the faucet.

If you are interested in getting this one for yourself you can buy it from Amazon. Please see the video below to understand better how the Python water changer hose works.

Using Canister Filter

Some aquarium owners who use canister filters use the filter to do the job for them. Simply bring the return tube out of the aquarium, and that way you can drain the water from the tank. You’ll need an extra hose for doing that.

After draining the water place the return tube in the aquarium again. Now, place the intake tube in the already prepared water, and fill the tank.

Use A Water Pump or Bucket

If you own a small aquarium and don’t use a canister filter or don’t have the budget to buy the special tool you can use a bucket. If you want to reduce the manual effort, use a water pump instead of lifting the bucket.

Take out the desired amount of water from the tank, and after that refill with the new water. Low-cost pumps are readily available in the local pet store or online. You can easily have one.

What Are The Recommended Water Conditioners For Water Change?

Seasoned aquarists use water conditioners in every water change. They use those on purpose. Water conditioners help to maintain the stability of the aquarium water after a water change.

It prevents sudden shock to the fish, in fact, works as a protector. Below are the most common water conditioners that fish keepers use to utilize during a water change.

Name Best For Dose Price Where To Buy
Seachem Prime Removes chlorine & Chloramine. Detoxify ammonia, nitrite, and heavy metals. 5 ml for 50 gallons 20$ for 500 ml Available On Amazon
Seachem Safe It’s a dry version of Shechem Prime, and even more concentrated 1.25 g per 300 gallons 12$ for 250 g Available On Amazon
API stress Coat Removes chlorine, and chloramine and detoxifies heavy metals. Reduces stress on fish, and replaces slime coat on fish. 15 ml per 30 gallons 7.8$ for 16 oz Available On Amazon

Always abide by the recommended dose. Overdosing won’t do any good to your fish and even can harm them.

How to do a full water change in a fish tank?

Can you do a 100 percent water change in your fish tank? The answer is you shouldn’t because a full water change can crash the entire tank cycle. You may potentially lose all the nitrifying bacteria present in the substrate, and filter media.

However, on a few occasions if your tank is severely contaminated, and there is a disease breakout you may prefer to do a 100% water change. So, how can you do that?

A 100% water change can be done in two ways:

  • Transfer all the fish to a quarantine or hospital tank for a while.
  • Keeping all the fish in the tank but changing 50% water at a time, and repeating the process.

Quarantine Tank Approach: Prepare the hospital tank with the required pH, temperature, etc. After that, transfer the fish from the main tank by slowly acclimatizing to the hospital tank water.

Then, you do your job of changing 100% water. Use a water conditioner, and also you can add some nitrifying bacteria. When the water change is done slowly transfer the fish to the main tank. Remember acclimatization is the key.

Repeat Water Change A Few Times: Many aquarists followed this approach with success. In this approach, you’ll change a 50% water first. After that, refill 50% of the water with new conditioned water.

Again change 50% of water, and repeat this 3-4 times. By doing this you’ll eventually end up changing almost 100% of the water without causing much stress to the fish.

However, never clean the filter or much of the substrates on the same day. In addition, never forget to use the conditioner.

Conditioners help in many ways, that help reduce fish stress, and remove chlorine and chloramine. Also, it detoxifies ammonia and replaces the protective fish coat.

Take away

Now you perceive that water change is not any dangerous business, but rather an absolutely essential part of fish keeping. You have to perform it in a way so that fish can adapt to the change. 

A sudden change in water chemistry is the primary cause of fish deaths after water changes. But fish deaths after a water change are avoidable.

The key that gives the best result for a water change eliminating associated risks is a routine.

When you follow a meticulous routine for water changes, your fish will be easily habituated to it. Good quality water can keep your fish healthy and lively with a longer lifespan.

No more heartbreaking scenes of dead fish floating in the aquarium! Do regular water changes and do it correctly. Water quality in an aquarium is uncompromisable.

Sujit Modak

2 thoughts on “How to Change Fish Tank Water Without Killing Fish | Be a Pro”

  1. Thanks! I really found this helpful! I was wondering… I have a 5-gallon tank, and once a week I try to replace about a gallon of water, is this fine to continue, or should I do less than that?

    • You are changing 20% of the water weekly. It is just perfect. But the fish density is another consideration. If you are keeping more than 3 fish in the tank, you should do one-third of water changes once a month. If possible, perform a nitrate test, and it will give you a better idea of the frequency of water changes you need.


Leave a Comment