Distilled Water For Fish: Is It Safe?

Distilled Water

Because distilled water is safe for human consumption and even considered beneficial, you might think that it is okay to use in your aquarium as well. However, distilled water is not always a wise choice when it comes to your fish. There are a number of factors involved. For one, fish tend to be happier and healthier when they aren’t swimming in water that was stripped of its mineral content.

However, that should not prevent you from keeping some distilled water handy for your aquarium, just in case. So, is using distilled water for fish safe? Let’s find out.

What Is Distilled Water?

For those who are confused or who generally do not know, distilled water is not like spring or mineral water. It is produced through a process called distillation, which is when water is heated to a boil then reduced to steam. Once the water is cooled and turns back to water, it is more pure, since minerals, such as iron or calcium, gases, and other contaminants, have been removed. 

Distilled water contains hydrogen and oxygen, and it has a pH level of 7.0.

Now, while this can be good for humans, since most of the time we get essential minerals from the food we eat, it is the opposite with fish.

What Happens When You Only Use Distilled Water For Fish?

When you decide to fill an aquarium with water, you have a number of choices. Tap water, well water, store-bought spring water, and so on. 

Which is what makes a lot of fish owners think, “Why can I just use the purest source of water available?”

You should always test the water to make sure it is clean and at the right pH level.

Which is what makes a lot of fish owners think, “Why can I just use the purest source of water available?”

Unfortunately, exclusively using distilled water is not going to do your tank and its residents any favors. Your fish need the vitamins and minerals dissolved in water. Further, the purity of the water can be upsetting and cause your fish a lot of stress. Remember, fish fins and gills are semi-permeable, and distilled water will not be enjoyable to them. Betta fish, in particular, can be traumatized if kept in distilled water.

Please don’t do that to your fish. They will not survive long.

How To Use Distilled Water For Fish Correctly

Think about where fish thrive in their natural habitat. Fish live in murky lakes, rivers, and ponds that get crowded with algae, dirt, mud, and other contaminants from rain. Wherever you find fish, you find water that isn’t 100 percent clean. While there are some uses of distilled water for fish, you mainly need to consider timing.

Here are some times when distilled water can be used in your aquarium:

Returning The Mineral Balance

Sometimes, water will evaporate from the fish tank. Returning momentarily to the process of distillation, you may remember that when water turns to vapor, it leaves impurities and minerals behind in the water that is left. In other words, if your fish tank loses too much water in a short period of time, the remaining water might be overloaded with minerals and not enough water molecules.

When this happens, it is perfectly acceptable to use distilled water to bring the level back to normal without disrupting the careful mineral balance you have achieved.

Reducing The Hardness of Tap Water

Do you only have a decent supply of tap water to fill the aquarium with? Chances are that the tap water you use is going to have a high level of hardness and pH. The best way to reduce those numbers and make tap water more hospitable to fish?

Add some distilled water to it. A little distilled water can cut hardness in half and will stabilize the pH value. This will save your aquarium from fluctuations that could negatively impact the plants and fish.

Of course, you will need to do more to tap water than just use distilled water. There are some chemicals that are added to tap water, such as chlorine and chloramine, that are harmful to fish. If you find the chemicals present, you will also need a water conditioner to detoxify it of heavy metals.

Changing or Cleaning Aquarium Water

Anyone who has experience with fish tanks will know that one mistake, one slight change in the equilibrium of the aquarium, could ruin everything. Your fish could die in the blink of an eye if you are not vigilant.

If you have fish that are susceptible to certain ills, you can use distilled water to create a pollutant-free home. All you have to do is re-mineralize, or add in the vital minerals, to the distilled water before reintroducing your fish.

Never forget: Your fishes need minerals like iron and calcium found in water to survive. So, you should never use distilled water without supplying it with enough minerals.

What Is The Safest Way of Using Distilled Water For Fish?

Based on the above discussion outlying the best ways to use distilled water, you know that it is possible but not necessary. Plus, you need to be ever mindful about how you integrate the distilled water into the aquarium. Far too many tank owners have not kept the above mentioned instances in mind and wound up either hurting or killing off their entire tank of fish.

Fish are highly sensitive to changes in their environment, especially in changes of pH. If you suddenly decide to switch their water, it could have negative effects.

So, if you are going to add distilled water to the aquarium, how do you do it without opening Pandora’s box? What is the safest possible method?

Here are some steps to take when using distilled water for fish:

  • Never use distilled water exclusively. Fish have selectively permeable membranous tissue that make up their gills, which allows them to breathe and absorb minerals and oxygen from the water. Placing fish in distilled water will hurt them, so don’t do it.
  • Make sure you are re-mineralizing the water. There are plenty of ways to add minerals back into water, such as adding natural plants, substrate, and mineral supplements available at pet and fish stores.
  • Combine distilled water with tap water that is chemical-free. You can top off a tank filled with tap or spring water with distilled water to balance out the mineral ratio. This would create a clean habitat for your fish.

Alternatives To Distilled Water For Fish Tank

Lastly, let’s touch on the best kind of water for your fish tank. You obviously want the cleanest, freshest source possible, one with minerals and plenty of air. Here are some alternatives to distilled water that are healthier and safer for your aquarium:

  • Purified water: This will need to be adapted to the needs of the fish and may require some re-mineralization and buffering. But at least you know it is clean.
  • Tap water: Also long as the tap water is chlorine-free, you can use it for your fish. However, you should always test for other toxic chemicals, nitrates, and confirm the levels of the gH, kH, and pH.
  • Reverse osmosis water: Sometimes simply called RO water, this water is usually recommended for Intermediate to Advanced level fish keepers, not beginners. The reason is that, once RO water is exposed to certain chemicals, its pH levels may drastically change. Therefore, one should learn how to balance pH before using it for their aquarium.


Hopefully, you now understand that using distilled water for fish is not the wisest choice. Water that is devoid of minerals is going to create a traumatic environment for your fish or even kill them. 

Therefore, you should always re-mineralize distilled water before using it or only have it on hand for the times when you need to dilute or rebalance minerals or other properties of the fish tank’s current water supply. If you add in minerals, however, distilled water can easily be one of the safest sources of water for your aquarium.

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