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 to thrive. If you want to keep fish as pets, you should be aware of what you’ll need. A healthy aquarium should contain the right kind of water to ensure your fish’s health.

This probably makes you wonder, is distilled water safe for a fish tank? In this article, we 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 so it returns to a liquid state. Distilled water is free of minerals such as irons and calcium, any gases, and 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. Where you lives affects the quality of your tap water. Seasonal changes, pollution, geological differences all impact water in different ways. Tap water generally undergoes chlorination to neutralize and filter foreign particles, bacteria, and other microorganisms. In comparison, distilled water undergoes boiling which removes heavy metals and nutrients like calcium and magnesium. Many of these minerals that are absent in distilled water are essential for your fish.

Is Distilled Water Safe For Your Fish Tank?

You should not flll up your fish tank with distilled water since it lacks many important nutrients. 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 live a healthy life 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 present 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. You should not fill your tank with distilled water, but it can work well to top off a full tank. If you have nothing except distilled water at hand, you should 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 to use and a great choice for your fish tank when remineralized. This means that you will need to add essential minerals yourself after filling up 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 at hand, you can use a combination of Epsom salt, calcium chloride, and baking soda instead. The amount of these materials will depend on the type of fish you have since all fish have different mineral requirements.

You can also use a remineralization cartridge but it is uncertain whether those actually 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.

If the water lost in your fish tank is due to evaporation, you can top off with distilled water safely. This won’t cause any harm to your fish because evaporation still leaves all the minerals behind. You can use distilled water to cover evaporation losses without compromising your fish’s health in any way.

As discussed above, 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 that is 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 and all you need to do is turn on your faucet. Many types of fish can actually live a long, healthy life in a tank full of tap water. However, there are some downsides and challenges that arise when using tap water. For starters, the quality of tap water can differ widely by area. Weather changes, water hardness, and location can all affect the quality of tap water.

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 about 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 are supplied tap water, the government does not regulate the quality of spring water. As a result, you will have to test it yourself to ensure that 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 the area you live in. You will have to adjust the hardness and test for impurities. At times, fertilizer can also seep into the water, adding harmful chemicals such as nitrates and phosphates. If the spring you collect water from is on a farm, there are chances this could happen.

Spring water isn’t the best choice and quality varies widely across regions. Compared to other types of water, it might be difficult to access spring 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 people suggest that RO water is actually the best choice for any aquarium. The filtration system effectively removes harmful bacteria and chemicals. It is essentially 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.

Deionized (DI) Water

Simple 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 conain chemical resin which 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. If you are limited to spring water, a deionization device can filter water to pesticides. The only downside to a DI system is that it cannot remove bacteria.

Similar to RO and distilled water, you need to remineralize DI water as well. The resin draw out all chemicals from the water. When using DI water, people also use and RO system to filter out 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 provides you the best of both systems by combining them. This system lets you effectively filter out bacteria, harmful chemicals, bacteria, and pesticides. The purified water from this systems contains little to no impurities. However, it still needs to undergo the remineralization process to be safe for your tank. This is the complete solution for those who are conscious about the water they drink. If you live in a state with poor quality water, an RO/DI system would be beneficial for you as well. RO/DI systems produce the purest water since they effectively filter all impurities.

When using RO/DI water, make sure to use a remineralizer. 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. Since humans usually purchase this type of water for drinking purposes, it can be expensive to fill up a tank with it. However, if you can afford it, purified mineral water is the best choice for your tank. Instead of having to buy remineralizer, you can just use this water to fill up your tank. You can also use an RO/DI system and then remineralize water for your fish tank yourself.

Natural Water

By now, 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 to it. 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 be suitable for your fish. That way, you =can skip experimenting till you find the best for your pet.


Keeping a dish tank is a big responsibility since you have to take care of multiple factors. The kind of 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 a variety of other options for your tank such as RO, DI, and tap water. A certain type of water can work well for a specific kind of fish. It is best to inquire at your local fish store about which type of water would be the best for your fish.

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