Distilled water for the aquarium: is it good to your aquarium?

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If you're a beginner, you may not be familiar with all types of aquarium water and how each will affect your aquarium. Good water chemistry is the basis for a healthy aquarium system, be it fresh water or salt water.

The main water options available for aquarium systems are tap water, well water, mineral water, reverse osmosis water (RO), reverse osmosis water (RO / DI), and finally, distilled water. This article specifically explains how the use of distilled water will affect your aquarium.

Read on to find out everything you need to know about the different types of tank water available, and how and when to use distilled water in your own freshwater or saltwater fishing tank!

What is distilled water?

Distilled water is water that has been processed to remove certain contaminants through a heating and cooling process. To distill water, the water must first be boiled. The generated steam is then cooled and collected, resulting in more purified water. While distillation removes many impurities, it does not remove those that have a lower boiling point than water; This means that some unwanted particles may still be present in the distilled water.

This process tries to mimic the natural events of evaporation, condensation, and precipitation.

How is distilled water different from tap water?

There are a few key differences between distilled water and tap water, the main difference being in nutrients and minerals.

The quality of your tap water depends on where you live. Tap water is taken from reservoirs, which are constantly affected by weather conditions, runoff and geological composition. Important water parameters such as the pH value and water hardness can differ greatly depending on the location due to the daily entrances and exits.

Tap water is treated in facilities primarily to filter and neutralize foreign particles, bacteria, viruses, parasites and other chemicals. These facilities often add chlorine or chloramine to best preserve the water and kill any remaining pathogens. However, this process allows many elements, nutrients / minerals, metals and other "aquarium contaminants" to remain in high levels.

In comparison, distilled water removes most of these other contaminants as long as they can be naturally boiled out. This means that the most dangerous heavy metals that pose a threat to your aquarium, such as lead and mercury, can be filtered out.

However, this also means that many beneficial nutrients and minerals such as magnesium, calcium and phosphate are also filtered out in the process. For many tanks, these minerals and nutrients are crucial for fish, invertebrates and plants to keep cell processes going.

Typically, hobbyists test these parameters individually and as Total Dissolved Solids (TDS) to get the best possible reading of their entire aquarium. The total dissolved solids measure all inorganic and organic substances in the aquarium water and are given in ppm (parts per million). TDS are important to the aquarium as they indicate the available resources for fish, plants, and invertebrates. This is also an important parameter that must be taken into account when changing the pH value, water hardness (GH) or carbonate hardness (KH).

Tap water vs. distilled water in freshwater fish tanks

In general, tanks planted with freshwater particularly need the natural minerals found in tap water in order to photosynthesize. Shrimp and other invertebrates also need these minerals in order to molt and grow properly. When hobbyists plan to use distilled water in their freshwater fish tank, they typically need to use other products to remineralize the water and adjust TDS and water hardness accordingly. While this gives more control over nutrient input and output, it can be expensive and difficult to assess at first.

This is in contrast to using a tap water source that already contains minerals and other important nutrients. However, these levels may still be insufficient or too high and may still need to be adjusted to keep the water safe for your fish, plants and invertebrates.

Tap water vs. distilled water in saltwater fish tanks

The water parameters of saltwater fishing tanks usually need to be slightly more accurate than the freshwater parameters, especially when maintaining a reef system. Many hobbyists try not to use tap water in saltwater aquarium systems because all of the unknown nutrients and minerals can be easily introduced.

In addition to the salt content, the most important parameters in salt water are typically the pH value, carbonate hardness, nitrates, phosphates and calcium. The problem with using tap water for saltwater fishing tanks is that these values ​​can fluctuate slightly each day, which can lead to instability. Most tap water also contains more phosphates than desired, which can quickly lead to algae problems.

In addition, tap water is known to introduce other important contaminants, such as lead from pipes, that can quickly destroy a marine aquarium. For this reason, most reefers and saltwater fish keepers want as much control as possible over their nutritional levels and choose another more reliable water source such as distilled water, reverse osmosis water, or deionized reverse osmosis water.

How is distilled water different from reverse osmosis water?

Another common option for aquarium water is reverse osmosis water (RO) or reverse osmosis water (RO / DI). There are some differences between the two, so knowing which one is best for your own tank is important as these water systems can be expensive to install and maintain.

Reverse Osmosis Water (RO)

Reverse osmosis removes particles similar to distillation, but uses a semi-permeable membrane to filter out molecules and other particles larger than water (H2O), such as metals and some minerals / nutrients. At the end of this process you will have safe and pure water with very low GH, KH and total TDS.

Reverse osmosis is usually used for both freshwater and saltwater fish tanks, as it can be adjusted to specific nutrient levels just like distilled water. Reverse osmosis water is typically used by hobbyists who plan to have their tank for a long time. This is because it is usually cheaper to buy and install a four-stage reverse osmosis system and keep up with changing membranes than it is to buy constantly distilled water.

Reverse osmosis deionized (RO / DI) water

RO / DI water is similar to normal RO water and also works on a semi-permeable membrane, but is subject to the additional step of deionization. This step results in even purer water with no TDS. RO / DI systems are typically five or six tier systems.

In short, the deionization stage uses positive and negative charges to remove any permanent ions that could have passed through the previous stages of the system. Many hobbyists believe that RO / DI systems are vital to reef tank success and that if your goal is to get the best water for your dollar, it is more than worth buying one. Just like using distilled water or RO water, you still need to add minerals to make the water safe for your fish, plants, and invertebrates.

Is RO water or RO / DI water better?

As you've probably heard, bigger is usually better in the aquarium hobby. For the most part, this also applies to aquarium equipment. If you spend the money and do the entire installation on an RO system, you can also spend the extra money on a full RO / DI system.

Technically, RO / DI water is much purer than RO because of the additional deionization step, but for the average aquarium, both make safe and ready-to-use water for fish.

When should you use distilled water in the aquarium?

With all of the different types of water available for your aquarium that all appear to be doing similar things, how do you know if you should be using distilled water for your aquarium or something else?

If you use distilled water you don't have to worry about unwanted nutrients or other metals / chemicals getting into the tank that may not make it safe for fish. However, this means that distilled water will be stripped of most of the essential minerals that many fish, plants, and invertebrates need to go through certain cellular processes, and it will likely be necessary to add back what has been taken out.

How to use distilled water

Many freshwater hobbyists like to mix distilled water with tap water or spring water to reduce the number of TDS put in the tank while maintaining the base value. Freshwater fish tanks can either be replenished with this mixture of distilled water and tap water for evaporation or used to perform a complete water change. However, if your tap water is not the best water to use for fish, you will need to remineralize the water, which may be worth investing in an RO system.

Likewise, many saltwater aquarium holders also use distilled water for refilling and for carrying out entire water changes. However, the main difference for saltwater aquariums is that when topping up, only fresh water is added, while when changing water, water needs to be mixed with salt to achieve the salinity level. This means that topping up with purely distilled water does not introduce or replace the essential minerals in the tank, but rather the water changes the water as the minerals and nutrients come from the salt mixture instead.

As mentioned earlier, most serious saltwater aquarium holders end up investing in an RO system as it is the best way to keep aquarium water safe for fish, the most adjustable and affordable way to do aquarium maintenance in the long run.

For any type of water, be it tap water, spring water, distilled water, RO or RO / DI water, or any other purified water, it is always important to have a way to remove chlorine independently. Even the best filtration systems may not filter out the chlorine that is typically added as a protective measure during treatment. All you have to do is add some water conditioner to your aquarium and it should be fine!

Should You Use Distilled Water For Your Betta Fish?

Again, distilled water lacks many of the essential minerals that fish, plants and invertebrates need for many bodily functions. This means you would have to put nutrients back into the water even if it was a Betta fish.

Many pet stores stock specialized purified water that is designed for use in betta aquariums. However, mixing distilled water and tap water should also be safe for your fish.

For a complete guide to setting up your first Betta Aquarium, check out our Ultimate Betta Guide here.


The type of water you use for your aquarium will largely depend on the plans you have for the aquarium. Most of all, you want to make sure that you are creating a safe environment for your fish, plants, and invertebrates. However, distilled water alone is typically not the absolute best water to use for your freshwater or saltwater tank.

In most cases, you'll need to remineralize distilled RO or RO / DI water to add back the minerals and nutrients you've taken. This can be done by mixing mineralized water with some type of purified water or by purchasing a remineralizer separately.

If you have any questions about distilled water, how does distilled water affect fish, or if you have been successful with any specific type of water in your aquarium, don't hesitate to leave a comment below!

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