Can I Use Hot Tap Water for my Aquarium? Can it Kill Fish?

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Numerous times I’ve been asking myself whether I can use relatively hot tap water for my fish tank. Since that is the most convenient way of adding new water to aquariums, I had the feeling that other aquarists ask themselves the same question. Now, after tons of research, I am willing to share my conclusions.

Hot tap water (130 degrees F and above) is dangerous for fish and should not be used in aquariums. It will create a temperature spike that will stress the fish and severely harm them. However, warm tap water, usually between 110 and 90 degrees F, can be used in fish tanks if properly dechlorinated.

As we move forward in this article, I will show you how to adjust tap water for fish tanks properly. Following these steps will create the perfect environment for your fish and increase their chances to live long, healthy lives.

Can I Use Hot Tap Water for my Aquarium?

A lot of aquarists use tap water in their aquariums. Tap water is not only easy to access, but it is predictable; that is to say, you know what to expect. A simple phone call to the relevant authorities in your area will tell you everything you need to know about the water’s chemistry coming through your tap.

For the most part, if you take the necessary precautions, you can eliminate the components that make tap water so dangerous, allowing your fish to swim through it without suffering any negative consequences.

However, some people cannot help but wonder whether the temperature matters. Every aquarist with experience understands the importance of eliminating toxins like chlorine from tap water before adding it to an aquarium. But does the temperature of that tap water make any difference? 

Well, it depends. Every species of fish has a particular temperature range within which it flourishes. To perform a water change, you must introduce new water to your fish tank, a process that can drastically change the parameters of the aquarium. 

That doesn’t include only the pH and hardness, but also the temperature. That is to say, if the new water is hot, it will raise the temperature in the aquarium. If it is cold, then it will lower the temperature. Fish hate such drastic shifts in temperature.

This is why beginners are encouraged to ensure that the new water is similar in temperature to the tank’s water before they add it. You can add warm water to your aquarium. In fact, the practice is encouraged because it will ensure that the new water and the old water share similar temperatures.[1]

Some people use thermometers to test their new water before adding it to the tank. However, most aquarists prefer to rely on their hands. That is to say, they will place a hand in the new water (held in a separate container), and then they will move that same hand to the water in the aquarium. 

If the two feel like they match, the aquarists will add the new water. If the new water is too hot, the aquarists will wait until it cools. This method sounds unpredictable and untrustworthy, but it works, especially for aquarists that trust their senses.

While warm water is acceptable, the same cannot be said for hot water. Hot water will cause the temperature to spike dangerously, harming your fish in the long run. It is even more dangerous to add boiling water to an aquarium. First of all, the process of boiling the water tends to transform its chemistry. 

The water that evaporates, as a result, will cause the concentration of the minerals in the remaining water to rise. The pH is also just as likely to change. Secondly, boiling water that is allowed to flow through copper pipes may introduce contaminants that will ruin your aquarium’s balance.

Can Tap Water Kill Fish?

Tap water can kill fish since it typically contains many contaminants that pose a danger to fish. The most problematic are chlorine and chloramine. They might burn the fish’s gills, compromising its ability to breathe.

It would help if you also kept an eye on the heavy metals. In the wrong concentrations, they will harm your fish, inducing stress, and making them vulnerable to diseases that will eventually kill them. Some old pipes will corrode, introducing copper and lead into your tank. This is why you are encouraged to treat tap water carefully.

How Long Should Tap Water Sit to Remove Chlorine for Fish?

Tap water should sit approximately 24 hours for chlorine removal. Since chlorine is unstable, it will most likely evaporate within one day. However, letting the water sit for 48 hours is the safest approach. After that period, the water should be adequate for the average fish.

If you are in a hurry, you can expedite this process by improving the aeration.[2] By adding air stones and pumps, you can increase the rate at which the chlorine evaporates. That will also oxygenate the water, turning it more suitable for your fish.

How do You Make Tap Water Safe for Fish?

Making tap water safe for fish involves these steps:

  1. Let the water sit for 24-48 hours to allow chlorine evaporation.
  2. Use a proper kit to test the water for ammonia and estimate its pH.
  3. Install a Reverse Osmosis filter to your plumbing to remove contaminants and viruses.
  4. Pour a stress coat water conditioner to provide the fish with additional disease protection.
  5. Measure the water hardness, and aim to 4-12 dGH for an average tropical fish tank.

Even though tap water is prevalent among aquarists, you cannot pour it directly into your aquarium, not without taking steps to treat it. While tap water is safe for humans to drink, it has several attributes that make it a danger to fish, for instance:

High Chlorine Concentration

That is the most obvious threat. People add chlorine to tap water to make it safe for consumption. The substance kills bacteria; however, chlorine is also toxic to fish.[3] It only takes a few milligrams to harm them severely.

Because chlorine is unstable, some authorities have replaced it with chloramine, which combines chlorine and ammonia. Nevertheless, not only is chloramine stronger than chlorine, but it is also more challenging to eliminate.

Heavy Metals

Tap water is home to heavy metals like copper and lead that fish did not appreciate. The concentration of heavy metals will depend on the source of the tap water. Some water bodies have greater concentrations than others.

Interestingly enough, heavy metals are beneficial to fish. For instance, zinc is vital to a fish’s biochemical functions.[4] Yet, excessive amounts of zinc will harm fish, and the same is true for other heavy metals. You have to maintain them in smaller concentrations to benefit your fish.

Water Hardness

The hardness is concerned with the concentration of minerals dissolved in the water. On that topic, it would be best to differentiate between General and Carbonate Hardness. The first is associated with magnesium and calcium, while the last is connected to carbonates and bicarbonates.[5]

Also, there are three primary reasons why the hardness of water matters:

  • Osmoregulation – The hardness, specifically the GH, will affect osmoregulation, the process that fish use to balance the salt and water quantities inside and outside. A fish’s ability to live in freshwater or saltwater is determined by the way it balances salt and water. The wrong hardness can cause an imbalance in fish, leading to stress and death.
  • Electrolytes – This term refers to the salts and minerals that a fish needs for everything from muscle growth to digestion. Because the hardness is concerned with the concentration of minerals in the water, you can probably see how it might affect the fish’s health. If the water is too soft, the minerals a particular fish requires will become scarce. As a result, its health will suffer. If the water is too hard, the abundance of minerals will also ruin the creature’s health.
  • pH – The carbonate hardness will affect your ability to maintain a stable pH in the tank. For instance, if the water is too soft, it will become quite challenging to maintain a high pH level.[6]

Different fish have different requirements where hardness is concerned. Some fish prefer soft water, while others have an affinity for hard water. Because tap water is typically saturated with so many potentially harmful substances, you are expected to make it safe by treating it before adding it to your tank. 

That involves the following:

1. Lowering Chlorine Levels

As was noted before, chlorine is unstable and eventually breaks down. If you leave tap water to stand, the chlorine will evaporate out. You can expedite the process by either aerating the water or adding a de-chlorinator.

If you are new to the topic, know that de-chlorinators are chemicals that actively eliminate chlorine. I personally recommend getting the API TAP Water Conditioner (link to Amazon). That affordable product allows me to use tap water without worrying about its original conditions. I also don’t have to spend so much time waiting for the chlorine to evaporate.

2. Taking Care of Chloramine

As was mentioned earlier, chloramine is a little more challenging to remove. It would be best not to expect it to evaporate from the water. De-chlorinators won’t work on it either. They will eliminate the chlorine, leaving ammonia behind. 

You can solve this problem by purchasing a water conditioner that removes the chlorine and then neutralizes the ammonia. To ensure that the water is in the proper state, please take a look at the API Aquarium Test Kit (link to Amazon). That bundle will measure the ammonia, nitrates, nitrite, and pH within minutes.

3. Using Reverse Osmosis Filters

A reverse osmosis filter is a device that uses pressure and a semipermeable membrane to remove contaminants such as chemicals and viruses from the water.[7] It will also eliminate heavy metals, taking care of the water hardness.

After using and testing dozens of RO filters, the one I’m sure you should get is the Aquatic Life RO Buddie Three Stage Reverse Osmosis (link to Amazon). Just connect it to your tap and let the magic happen. Frankly, I do not doubt that my fish live long thanks to this device:

4. General Water Conditioning

Besides de-chlorinators, there are other chemicals and products that you can add to water to make it safe for fish. The types of water conditioners you buy will depend on the objective you want to achieve, for instance:

  • Stress Coat – Certain conditioners will eliminate chlorine and chloramine and regenerate sick fish’s skin, allowing them to defend themselves against further damage (including parasites and diseases). The conditioner does this by producing an artificial coating to replace the fish’s lost natural one.
  • Soft Water – If the water is too soft, you can use a water remineralizer, wonder shells, crushed coral, and aragonite, to mention but a few, to replenish the mineral content.
  • Hard Water – If the water is too hard, you can make it softer through the use of peat, driftwood, and water-softening pillows. As was mentioned before, reverse osmosis is quite useful. Though it might be easier to use distilled water.[8]
  • Heavy Metals – Besides reverse osmosis, you can remove heavy metals from the water using activated carbon, chelating agents, and poly filters.


Tap water that is too hot is not suitable for fish tanks. If the temperature is above 130 degrees F, the new water will create a dangerous spike that will stress your fish and possibly kill them. To avoid that, aim to lower temperatures, preferably similar to what your tank currently features.

If you wish to dechlorinate the water beforehand, all you have to do is wait 48 hours. That will allow the chlorine to evaporate naturally. You may also use products such as Reverse Osmosis filters or active de-chlorinators to make the water safe for fish more quickly.