Can You Put a Water Conditioner With Fish in the Tank?

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As an aquarist, numerous times, I have conditioned water for my fish. Naturally, I found water conditioners to be the most convenient method. However, I wasn’t sure if I should put the conditioner while the fish are in the tank. After extensive research, I am willing to share all the lessons that I’ve learned.

You can put a water conditioner while the fish are in the tank since water conditioners are generally safe to use. However, it would be best to remove the fish in small and overcrowded tanks for at least 15 minutes. That will allow the conditioner to diffuse equally across the entire aquarium.

As we move forward, I will share a few tricks to condition your tank correctly, using an aquarium conditioner. I will also show you how to use boiled water for the same purpose. That trick and a few others will allow you to condition your water even when your current conditioner runs out.

Can You Put a Water Conditioner With Fish in the Tank?

Most aquarists understand the importance of performing regular water changes. That is the only way to keep the tank free of toxins and contaminants. However, by introducing new water to an aquarium via a water change, you also risk adding toxins and contaminants.

That is where water conditioners enter the picture. You can use them to eliminate ammonia, chlorine, chloramine, and any metals that might threaten your fish. However, it is less clear how you should handle your fish while using these products.

Two primary reasons incite questions like this. First, because water conditioners are chemicals, some people worry that the products might harm their fish. So, they believe that fish should be removed from the tank before the water is conditioned.

Second, because water conditioners are used to remove toxins from new water, some aquarists worry that the toxins will do irreversible damage to their fish before the water conditioners can remove them.

These concerns sound valid, but they are mostly unfounded for two reasons:

  • Most water conditioners are safe and do not harm fish. A water conditioner that is dangerous to fish will warn you, which is why I recommend reading the instructions on the packaging. Yet, most water conditioners will guarantee the safety of your fish.
  • The best water conditioners, such as API Water Conditioner (link to Amazon), work instantly.[1] That is why some people condition new water while simultaneously adding it to the tank; they know that these products work in minutes. The conditioners will swiftly neutralize the toxins before they can harm your fish.

Therefore, you don’t have to remove your fish from the tank to add the water conditioner. However, you are free to take any precautions that you feel are necessary to protect your fish. That includes taking them out of the water before you add the conditioner.

How Long After Adding a Water Conditioner Can I Add the Fish?

Fish can be introduced to the tank fifteen minutes after treating the water with a conditioner. That is especially true for old tanks that don’t require cycling. However, usually, fish can be added to the tank immediately after using a conditioner or even stay inside throughout the entire process.

It is worth reiterating the fact that you don’t have to remove fish to condition the water. That being said, some people prefer to keep their fish in a separate container while they change and condition the water. Ultimately the waiting period will depend on your situation:

Old Tanks

In most cases, it shouldn’t take more than fifteen minutes for the conditioner to remove toxins from new water, at which point you can put the fish back, knowing full well that their water is safe. That assumes that you have an older tank that has been cycled.

New Tanks

If your tank is new, you are still expected to treat the tap water with water conditioners. Though, in this case, all you need is a product that removes the chlorine, leaving the ammonia behind for the bacteria to consume.[2] 

After conditioning the water, you have to wait a month for the tank to cycle. Water conditioners don’t change anything here and cannot hasten the process. Every new tank requires cycling before adding the fish.

Can Water Conditioners Kill Fish?

Water conditioners can kill fish, especially in concentrations above five times the recommended dose. That typically happens in small and overcrowded tanks or when the product is mishandled. However, in general, water conditions are considered safe for fish.

As was noted before, water conditioners are safe.[3] That is why you can add them to a tank without removing the fish. Naturally, the amount matters. Some conditioners must be added in small amounts because they are so concentrated. Others must be added in larger amounts because they are more diluted.

Smaller tanks are also tricky because you could easily add too much conditioner. However, even in such cases, the best brands have claimed that you can add two, three, or even five times the recommended dose of conditioner, and it won’t harm the fish.[4] Therefore, even if an accident or a spill occurs, the chances of a water conditioner killing your fish are low.

Why is a Water Conditioner Necessary for my Fish

Water conditioners are commercial products that aquarists add to aquariums to remove toxins. The first significant water conditioner was sodium thiosulfate.[5] It wasn’t the only product purported to combat chlorine, although it was the first to deliver truly revolutionary results without harming fish. 

Sodium thiosulfate could eliminate chlorine instantly. However, when chloramine was introduced, aquarists realized that sodium thiosulfate was no longer sufficient. It could still remove the chlorine, but it left the ammonia behind. Today, the market is filled with water conditioners that can combat chloramine. They remove the chlorine and then neutralize the ammonia. 

Water conditioners are necessary because most aquarists get their water from a tap. Tap water is easily accessible, although it is also treated with chemicals to make it safe for human consumption. 

Initially, tap water was only treated with chlorine. But this did not make it any less dangerous. Chlorine is a toxic substance that must be kept in concentrations of 0.003ppm or lower in an aquarium.[6] Trace amounts will induce stress in fish, and concentrations as minute as 0.2ppm will even kill aquarium fish.

Despite the danger it posed, chlorine wasn’t necessarily a concern for fish experts because it eventually evaporates. But then it became reasonably commonplace for governments to treat their tap water with chloramine, a combination of chlorine and ammonia. And unlike chlorine, chloramine doesn’t simply evaporate.

How do You Condition Water For Fish?

Conditioning water for fish involves these steps:

  1. Boil the water for at least 8 minutes.
  2. Let the water sit for approximately 24 hours.
  3. Introduce a water conditioner in a separate container.
  4. Let the fish sit in that container for a while before putting them in the aquarium.
  5. Use a biological filter containing sodium thiosulfate in the community tank.

If you are new to aquariums, you should keep the following in mind when conditioning water for your fish. These steps will ensure that your fish are not exposed to unnecessary ammonia and chlorine presented in regular tap water:

1. Ways to Condition Aquarium Water Without Conditioners

While water conditioners simplify the removal of toxins, there are ways to condition water without using these products. For instance:


You can eliminate chlorine by allowing the water to sit. The substance is unstable. Therefore, given enough time, it will eventually break down, evaporating from the water. In many cases, leaving water to stand for 24 hours will rid it of dangerous concentrations of chlorine.

You can expedite this process by adding filters and air stones to improve aeration and circulation. However, as was noted before, this won’t work on chloramine. The toxin is, unfortunately, more stable than chlorine.


If you don’t want to wait, try boiling the water. It can take eight minutes to remove chlorine from a 10-gallon tank.[7] Boiling also works on chloramine, though it takes longer. For instance, while you can remove 1ppm of chlorine from 10 gallons in 4 minutes via boiling, it will take 65 minutes to remove a similar amount of chloramine.

Biological Filter

This method involves the use of a de-chlorinator such as sodium thiosulfate. Add the de-chlorinator to a container of water to remove the chlorine. After this, you can connect the container to a biofilter that will turn the ammonia left behind into nitrate.

However, the downside with these methods is that it takes a while to produce results. That is why people prefer water conditioners. Personally, I prefer using conditioners, and I don’t worry too much about those harming my fish.

2. Conditioning Aquariums With a Water Conditioner

You can use one of two methods to condition your water using water conditioners:


If you usually use a separate container to add water to the aquarium, treat the container’s water before you add it. Acquire a conditioner, fill the container with water, measure the conditioner, and add it to the container’s new water. It would help if you also took a moment to swirl it before pouring it into the aquarium.


If you use a hose to perform water changes, you can add the water conditioner to the tank even as the new water enters the aquarium. With the container option, you only need enough conditioner to treat the water in the container.

With the hose option, you must identify the conditioner’s right amount required to treat the entire aquarium. Once you measure the conditioner, slowly drip it into the water as it streams into the aquarium.[8]

Either way, if you feel unsure, bear in mind that water conditioners have instructions to educate you on the right amount to use. I also recommend watching some Youtube videos that provide some visual understanding.

Water conditioners must be used whenever you add water to the tank. In other words, every time you perform a water change. Basically, all new water must be conditioned before you add it to the aquarium.

What Types of Water Conditioners Should I Consider?

There are three primary types of water conditions for you to choose from, namely:[9]

  • De-Chlorinators – As the name suggests, these conditioners remove chlorine. They will ignore the ammonia and heavy metals. People don’t use them as often because they prefer products that can attack both chlorine and ammonia.
  • Chloramine Neutralizers – These products are also aptly named. They neutralize the chloramine in the water by eliminating the chlorine and then binding to the ammonia molecules, producing ammonium that does not kill fish.
  • Complete Conditioners – These products do a little bit of everything. They will attack the chlorine and ammonia while also removing heavy metals and copper. You can also trust them to buffer the pH and protect the slime coats that fish use to repel infections.

Before you can condition your water, you need to identify the types of conditioners you will use. If you wish to save yourself some trouble, I would highly recommend getting the API TAP Water Conditioner (link to Amazon). I never had any issues with that product, and it is impressively cheap.

You can use one or multiple conditioners depending on your objective. Try to read the label of every conditioner before you buy or use it. That will give you an idea of what they can do for you and whether they are sufficient for the tasks you have in mind. 

For instance, some products are categorized as chloramine neutralizers even though they combat heavy metals. Therefore, getting those means that you don’t have to invest in a particular product to neutralize heavy metals.

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Most water conditioners don’t require you to remove the fish. You can pour the right amounts into the existing tank while the fish are inside. However, small tanks require extra caution. That is to avoid elevated conditioner concentrations.

If you wish to avoid commercial products, you may also boil the water for conditioning. Letting the water boil for at least 8 minutes will take care of the ammonia and chlorine presented in tap water. Either way, make sure that you let the water sit for at least 24 hours before introducing it to your fish.