Is Distilled Water Safe For A Fish Tank?

There is a common misconception that fish are low-maintenance pets. However, for fish to thrive and grow to their full size, they need optimal living conditions. Enough space to swim around, food, a clean tank, and the right water conditions are just a few things fish need. If you want to keep fish as pets, you should know what your fish need. A healthy aquarium needs water that reflects the conditions of your fish’s native environment.

You may wonder if distilled water is safe for a fish tank? After all, distilled water is a form of purified water, so it doesn’t contain anything bad. Right?

This article will discuss the types of water you can use for an aquarium and whether distilled water is safe for your fish tank.

What Is Distilled Water?

Distilled water is a type of purified water, meaning it has undergone a mechanical or chemical process to remove impurities. The distillation process involves heating water till it boils and turns to steam, then cooling the steam to return to a liquid state. Distilled water is free of minerals like iron and calcium and any gases or other elements that contaminate water. It is perfectly safe for drinking but is not the best choice for a fish tank.

How Does Distilled Water Differ From Tap Water?

Distilled water differs from tap water mainly in nutrient content. Your location affects the quality of your tap water—seasonal changes, pollution, and geological differences all impact water in different ways.

Tap water generally undergoes chlorination to neutralize and filter foreign particles, bacteria, and other microorganisms. Distilled water is boiled to remove metals and nutrients like calcium and magnesium. Many of these minerals absent in distilled water are essential for your fish.

Is Distilled Water Safe For Your Fish Tank?

Distilled water lacks many important nutrients your fish need. The minerals in water are essential for the health of your fish. Fish also have semi-permeable membranes, so distilled water can disturb the chemical balance within a fish’s body. When osmosis occurs, it dilutes the bodily fluids of the fish, leading to lower concentrations of essential minerals. Fish cannot thrive in distilled water, and some die within a few months.

Another way to look at it is the Total Dissolved Solids (TDS) rating of distilled water. TDS refers to organic and inorganic elements present in water that indicate its pH level and hardiness. It also gives you an idea of the minerals in the water and whether they are in sufficient quantities for fish. Distilled water has the lowest TDS rating since it contains the fewest minerals compared to water treated through other methods.

Some types of fish, such as Betta fish, are very sensitive to distilled water. Never fill your tank completely with distilled water, but if you need to top it off a bit, you can make distilled water work, provided you add minerals to ensure the health of your fish.

How To Safely Use Distilled Water In Your Fish Tank

If you have no choice but to use distilled water in your fish tank, it is important to remineralize it. Distilled water is safe and a great choice for your fish tank when remineralized—meaning you will need to add essential minerals yourself after filling the tank.

Aquarium Sphere recommends using a mixture of water conditioner, baking soda, and acid buffer to remineralize your tank. If you don’t have these materials, you can use a combination of Epsom salt, calcium chloride, and baking soda instead. The amount of these materials will depend on your fish type since all fish have different mineral requirements.

You can also use a remineralization cartridge, but it is uncertain whether those work. The best option is to use the remineralization mix after filling up your tank.

When To Use Distilled Water In A Fish Tank

There are three instances when you can safely use distilled water in your fish tank.

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Water lost in your fish tank due to evaporation is safely replaced with distilled water. It won’t cause any harm to your fish because evaporation still leaves the minerals behind. Distilled water can cover evaporation losses without compromising your fish’s health.

Distilled water works well for an aquarium when remineralized. If it meets all the mineral requirements necessary for your fish, you can add distilled water to your fish tank. Remineralized distilled water makes a great choice for many aquariums.

You can use distilled water to adjust alkalinity and stabilize the pH. Since it contains no minerals, it can reduce carbon hardness in other types of water.

Other Types Of Water For Your Fish Tank

You have a variety of options to choose from other than distilled water. The type of water best for your fish tank depends on its TDS rating and the kind of fish you own. The different types of water include:

Tap Water

Tap water is the most convenient alternative to distilled water. It is more affordable than distilled water; all you need to do is turn on your faucet. Many types of fish can live a long, healthy life in a tank full of tap water. However, some downsides and challenges arise when using tap water. For starters, the quality of tap water can differ widely by area. Weather changes, water hardness, and location can affect tap water quality.

Tap water can also contain chlorine which is unsafe for fish. However, you can easily fix this issue by using a water conditioner. To learn more about the quality of your tap water, you can test it yourself or ask your local fish store. They can even tell you which fish thrive in tap water if you don’t want to spend extra money buying water for your aquarium.

Spring Water

Spring water, or well water, comes from underground sources and is untreated. The quality of spring water also varies by area. Since most homes have tap water, the government does not regulate spring water quality. As a result, you will have to test it yourself to ensure it is a good fit for your fish.

There are some disadvantages to using spring water in your fish tank. First of all, the quality of spring water varies largely by area. You will have to adjust the hardness and test for impurities. At times, fertilizer can seep into the water, adding harmful chemicals such as nitrates and phosphates. If the spring you collect water from is on a farm, this could happen.

Spring water isn’t the best choice, and quality varies widely across regions. It might be difficult to access spring water compared to other types of water. If you live on a farm, it is best not to use spring water since it can contain chemicals toxic to fish. Tap water can be a good alternative to spring water when dechlorinated.

Reverse Osmosis (RO) Water

Reverse Osmosis (RO) water is purified water obtained through mechanical filtration. An RO tank filters water through a super-fine filter to remove impurities. The holes in the filter are so tiny that only water can pass through. Minerals and bacteria are too large to pass through, so they are effectively filtered out.

The downside to RO water is that it lacks essential minerals. The filtration process removes any minerals, even the ones you need. However, you can easily add these minerals back to the water through remineralization.

Reverse Osmosis (RO) systems can be expensive to install and the filters require replacement every few months or years depending on the water quality in your area. RO water can be a great choice for freshwater fish after remineralization.

Many suggest that RO water is the best choice for any aquarium. The filtration system effectively removes harmful bacteria and chemicals. It is a “blank slate” that you can adjust fully according to your fish’s needs. Whether you have freshwater or saltwater fish, you can use RO water for both. RO water has the least amount of impurities compared to other filtration systems.

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Deionized (DI) Water

Similar to an RO system, you can also use Deionization devices to purify water. The system works better at filtering water through unique technology. The filters in a deionization device contain a chemical resin that can draw out impurities. The resin works to chemically filter water by drawing impurities in exchange for healthy minerals. They can remove harsh chemicals such as pesticides as well. A deionization device can filter pesticides from water if you are limited to spring water. The only downside to a DI system is that it cannot remove bacteria.

Similar to RO and distilled water, you must also remineralize DI water. The resin draws out all chemicals from the water. When using DI water, people also use an RO system to filter bacteria. You can use DI water for a fish tank after remineralizing it.

A Deionization system also requires set-up and regular maintenance. It is not as easily accessible as tap water. You will need to change the filters after a few years. Many people get the most out of their DI device alongside an RO device.

RO/DI Water

An RO/DI system gives you the best of both systems by combining them. This system lets you effectively filter out harmful chemicals, bacteria, and pesticides. The purified water from this system contains little to no impurities. However, it still needs to undergo remineralization to be safe for your tank. RO/DI is the complete solution for those conscious about the water they drink. If you live in a state with poor quality water, an RO/DI system would also benefit you. RO/DI systems produce the purest water since they effectively filter all impurities.

When using RO/DI water, make sure to remineralize. The water lacks the essential chemicals your fish need to thrive.

Purified Mineral Water

Purified mineral water is the perfect choice for your aquarium but can be more expensive. Purified mineral water is used mostly for drinking purposes, it can be expensive to fill up a tank with it. However, purified mineral water is the best choice for your tank if you can afford it. Instead of buying a remineralizer, you can just use this water to fill your tank. You can also use an RO/DI system and then remineralize water for your fish tank.

Natural Water

You’re probably wondering if getting water from a nearby lake is your best option. You might have seen fish in the nearby lake and assumed it could be safe for your fish. However, that is unfortunately not the case. Fish living in a particular body of water take a long time to adapt. All types of fish need differences in pH levels and certain minerals more than others.

If you’ve bought your fish at the local fish store, it is best to inquire about what water would suit your fish. That way, you can skip experimenting till you find the best for your pet.


Keeping a fish tank is a big responsibility since you must take care of multiple factors. The water you use in your tank can significantly impact your fish’s health. Distilled water is the purest form of water and can be a great choice for your tank if remineralized.

However, you also have many other options for your tank, such as RO, DI, and tap water. A certain type of water can work well for a specific fish. It is best to inquire at your local fish store about which type of water is best for your fish.

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Susan Dorling

I am a pet expert with years of experience working with a variety of animals. From dogs and cats to birds and exotics, I have a deep understanding of their unique needs and behaviors. I am dedicated to helping pet owners provide the best care for their furry friend.

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