Is Borax Harmful for Fish? Here’s What You Need to Know

If you’re an avid fish keeper, you may have heard of using borax as a cleaning product. This chemical is also a common ingredient in PH buffers for you tank’s water, as well as other product for aquariums… but is Borax harmful for your fish? This is what we’re going to discuss in today’s article.

First, we will learn what borax is, then we’ll move on to see if it is dangerous for fish or not, how to prevent borax poisoning and basically everything else that you need to know about this chemical.

What is borax?

Borax (sodium tetraborate) is a white, powdery chemical that looks similar to salt or sugar. It’s commonly used in laundry detergent, as well as household cleaning products, but for aquarists it is used as a water softener or buffering agent.

There are also trace amounts of borax in various other products that we use in our house or for the maintenance of our fish tanks.

Is borax harmful for aquarium fish?

fish in aquarium

Borax isn’t harmful for your fish when it’s used properly. The chemical can actually be beneficial for your fish and especially for the quality of your fish tank’s water.

For example, if you add a small amount of borax to your tank’s water, it will help keep algae growth down. It will also help to lower the pH level of your water, which is important for keeping your fish healthy.

However, you should never add too much borax to your tank. Too much of this chemical can be harmful to your fish.

So.. what does “too much” mean?

First of all, you should always double check the labels and use as instructed. But according to a study that I found, concentrations of less than 10 mg per liter are considered not to be harmful to fish.

How does borax affect aquarium fish?

Borax can cause various problems for your fish if it’s ingested. And since it would be all over the water that they swim in, it will automatically be ingested – hence the need to only have it in very small amounts (if any).

In addition, borax can also accumulate in your fish’s tissues over time and it can affect their breathing, but also their heart and nervous system.

In larger doses, borax can be very bad for the health of your fish. Exposure to higher doses can lead to your fish death in as low as 96 hours.

So, if you’re using borax in your aquarium, it’s important to be very careful not to overdo it. And make sure to check the label before using it so that you don’t accidentally poison your fish.

What are the symptoms of borax poisoning in fish?

The most common symptoms of borax poisoning in fish are lethargy, lack of appetite, and difficulty breathing. You will see your fish breathing harder, as if choking.

How can I prevent borax poisoning in aquarium fish?

The best way to prevent borax poisoning in your fish is to use it in very small amounts and to always double check the label. Never try to make your own products using this chemical and try to avoid using it completely if it’s not really a must to put it in the water.

And of course, make sure to store it safely, away from the water, so that it doesn’t get in by accident.

I personally never used borax in any of my tanks and everything went well. Usually, with proper maintenance, you can get rid of unwanted algae if that’s what bothers you, while increasing the PH level of your water can be also achieved through more natural means.

What should I do if I think my fish have been poisoned by borax?

If you think your fish have been poisoned by borax, you should act very quickly, as borax poisoning in fish can become lethal really fast.

The first thing you should do would be to quickly do a partial water change, replacing around 50% of the water. After a couple of days, perform a new partial water change of around 25%.

Make sure to also remove the filter and clean the sponge or active material, as that’s where much of the borax can end up. If using a sponge, you can even consider replacing it altogether.

During this time, you should stop using any products containing borax.


As you can see, borax is a potentially harmful chemical that you should use with caution.

Be sure to check the label before using it, and to only use it in very small amounts if necessary. And, of course, make sure to store it safely away from the water.

I have to repeat that I have never used (at least not intentionally) any fish products containing borax and I can only assume that you can skip using it altogether. Better safe than sorry.

Share the article below:

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.