Why Is My Betta Acting Weird After Water Change? (Solved!)

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If there is one thing that sets Betta keepers apart from other fish keepers, it’s the fact that we really get to know our Bettas.  To us, Bettas aren’t just another fish, they are pets.  Because we get to know our Bettas so well, we notice when their behavior changes.  

There are a number of reasons Bettas act weirdly after a water change and these include a rapid change in water temperature, sudden change of pH, and failure to dechlorinate the new water properly.

Why Do Betta Fish Act Weird After a Water Change?​

Water changes are an essential part of fish keeping.  Making sure we keep our Bettas water fresh by performing this routine piece of maintenance is extremely important.

Betta fish can deal with small changes in water parameters, but they struggle to cope when they are subjected to a sudden, dramatic change.

The reasons Bettas act weirdly after a water change include;

  • Drop in water temperature
  • Change of pH
  • Failure to dechlorinate properly
  • Bacteria crash

Drop in the water temperature

A sudden, dramatic drop in water temperature is the most likely cause of a Betta acting weirdly following a water change.

Bettas, like all fish, are cold-blooded and can not regulate their own body temperature. They are completely reliant on their environment to keep their bodies at the right temperature.

Bettas like their aquarium water to be around 78°F (25°C), and whilst they can handle temperatures just above and just below this point, they struggle if the water temperature changes massively, especially if it does it suddenly, as in the case of a water change.

The problem with water changing a Betta tank is we typically keep them in very small volumes of water.  A 10 gallon (38 liters) tank would be considered large for a single betta, and there are plenty of people who keep their Bettas in just 2.5 gallons or even less.

With such a small amount of water, the temperature can swing hugely during a water change.

To avoid a sudden drop in water temperature, consider either removing your Betta from its tank during the water change, temporarily housing it in another container until the water change is complete and the tank has come up to temperature, or make sure the water you are adding back into the aquarium is the same, or very close to the temperature of the aquarium water.

A sudden change of water pH

pH is the measurement of how acidic water is.  Bettas like their pH to be anywhere between 6.5 and 7.8, and they can probably handle slightly higher and slightly lower.

As with water temperature, Bettas may struggle when their pH swings suddenly one way or the other.  

If we don’t carry out regular water changes in our aquariums, the pH can drop over time due to natural chemical processes in the aquarium.  It is possible that your Betta tank started at a pH of say 7.5, which matches your tap water, then over the course of 6 or 12 months, the pH drops as low as 6.5.  If we were then to carry out a large water change after 12 months, the pH would rapidly change to nearer 7.5.  This sudden change could have a dramatic effect on your Bettas behavior.

If you are unsure of the pH of your Bettas tank, I would strongly recommend testing it using an aquarium test kit.  Personally, I use the Freshwater Master Test Kit which is made by API.  It is fairly accurate and simple to use. I just bought this one on Amazon

It is really important to test aquarium water regularly.  The video below discusses water testing in some detail.

Failing to dechlorinate properly

Most municipal water supplies are treated with either chlorine or chloramine to make the water safe for humans to drink.  The water has to stay free of bacteria until it reaches our homes. The problem is, both chlorine and chloramine are toxic to fish.  

When we carry out a water change it is extremely important we dechlorinate the water properly.

There are different ways to dechlorinate water, but all require a good quality chemical dechlorinator.  I have used Seachem Prime for as long as I can remember, but there are other brands on the market. 

When I carry out a water change, I usually replace the water using a bucket, and I will add the correct amount of dechlorinator to each bucket of water, stirring well before adding back to my Betta tank.  Others just add the dechlorinator to the tank after they have added the new water.

Whichever method you use, make sure it works for your Betta.

Bacteria Crash

Our Betta aquariums rely on beneficial bacteria (sometimes called good bacteria) to convert our Bettas waste from very toxic ammonia to far less toxic nitrate.  This bacteria lives on every surface of our Betta tank but is especially prevalent in the filter and on the substrate. 

Sometimes, when we carry out a water change, we also gravel vacuum the substrate and clean the filter.  This creates a perfect storm where we have disturbed all the major sources of bacteria in the Bettas aquarium.  

Following the water change,  our Bettas are left living in an aquarium that can’t handle the Bettas waste and the amount of Ammonia in the water causes the Betta to act weirdly.

In this situation, the best action to take is to stop feeding the Betta for 3 or 4 days.  The Betta will be fine but will produce considerably less waste, and the bacteria in the aquarium and filter will have a chance to multiple back-to numbers that can easily process the Bettas waste.

How To Perform A Water Change Without Making Betta Act Weirdly​

Whenever I water change my Bettas, I try and follow the same 10 step routine. By using my own tried and tested method, I seriously reduce the chances of affecting my Bettas or their behavior.

When you need to water change your Bettas, follow this plan and your Bettas will never act weirdly following a water change again.

1. Fill a small container with water from the Bettas aquarium.  This container will provide a safe place for the Betta whilst you carry out the aquarium maintenance.

2. Move the Betta to its temporarily holding container.  Make sure it can’t jump out of the container or get knocked over.

3. Using a non-scratch sponge, clean the inside of the tank of algae.  (Personally I only clean the front glass as algae is surprisingly beneficial in an aquarium, but that discussion is for another article!)

4. Drain down the aquarium by about 50% (assuming it is a relatively small tank, I wouldn’t drain anything over 10 gallons by as much as 50%).  If necessary, gravel vacuum whilst draining the water out.

5. Fill a 5 gallon (20 liters) bucket with water, getting it as close to the aquarium water temperature as you can.

6. Add an appropriate amount of dechlorinator to the bucket of water.  I use Seachem Prime, so I only add 1 to 2 drops for every bucket of water.  Stir well before adding to the Betta tank.

7. Carefully refill the Bettas tank with the dechlorinated water, taking care not to disturb the substrate, plants, and decorations.

8. Once the tank is refilled, check the filter and heater are working properly.

9. Leave the tank for sufficient time for the temperature to come back up to the right point. I have my Betta tanks set to 78°F, so that’s the point I am looking for.

10. If all is well, return your Betta to its aquarium.

Some Frequently Asked Questions About Water Changing A Betta Tank​

Q: How often should you water change a Betta?

A: There are no hard and fast rules about how often a Bettas tank should be water changed.  In my experience, carrying out a water change once every 1 to 2 weeks works well.  

Q: How long can a Betta go without a water change?

A: There are a number of variables that would go into working out how long an individual Betta could go without a water change.  If you own an aquarium test kit, you can test the water on a regular basis, then only carry out a water change once the test kit results become unacceptable. 

Q: Can I do a full water change on a Betta tank?

A: Changing 100% of the water isn’t ideal, but can be done. Use the same technique as you would for a 50% change but consider not feeding your Betta for 3 or 4 days after the water change to allow sufficient beneficial bacteria to recolonize the aquarium.

My Final Thoughts On ‘Why Is My Betta Acting Weird After A Water Change?’​

Bettas can occasionally act weirdly after a water change.  The most likely reason is due to a sudden change in water temperature.

Other reasons include a change of pH, insufficient dechlorinator being added, or because we inadvertently killed off the majority of the beneficial bacteria in the aquarium. 

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James