With advanced technology, the options available for setting up and maintaining aquariums have changed drastically. There are multiple choices about everything you can add to the aquarium.

Water is one of the most crucial elements for fishes to thrive. Are you confused about the best aquarium water for your fishes? Will that be easy on your pocket and less time-consuming?

This article examines using distilled water in fish tanks. Its pros and cons and ways to make it safe for your fishes. We shall briefly discuss the types of water sources so you can select the best water for your fish tank.

Requirements of Aquarium Water

Let us begin by understanding what makes aquarium water different from the water we use and the importance of having the correct water for your fishes.

We utilize water from the various sources listed below.

  • Well-water
  • Rainwater
  • Spring water
  • Tap water
  • Mineral water
  • Bottled water
  • Distilled water
  • Reverse osmosis (RO) water
  • De-ionized (DI) water, etc., to name a few.

Choice(s) vary according to the purpose, availability, accessibility, and socio-economic factors.

We derive essential nutrients from both – food and water.  Nutritional deficiency from the water gets covered by the food. Besides, we also take specific vitamin tablets regularly, if need be.

Against this, water to fishes is what air is to us. It is their life. They use nutrients present in the water to survive and thrive. Their food may not have all the minerals they need.

Besides, several minerals which are non-toxic for humans (like copper, chlorine, etc.) are toxic for the fishes.

Another impacting factor is the original habitat v/s the captive location. The two are geographically apart due to local and international fish trade. Hence, adjusting the water quality becomes vital. Local fishes will often survive in local water without much ado.

Hence, none of the above types of water can be used directly for fish tanks. We test it. Fix the water quality and then add it to fish tanks.

Once the tank matures, it has an established ecosystem with good bacteria to offset the harmful chemicals. However, we need to perform partial water changes regularly to maintain the water quality.

Various parameters to test aquarium water are

  • PH (Power of hydrogen) – It indicates the acidic or alkaline content. Higher PH means more alkalinity. PH has to be stable for a good aquarium. Any changes need to be gradual. Fluctuating PH can be fatal to tank dwellers.
  • GH (General Hardness) – It denotes the calcium and magnesium in ppm (parts per million) or mg/L (milligrams per liter) or dGH (degrees of General Hardness). Higher GH means hard water. Marine water is hard water.
  • KH (Carbonate Hardness) – The presence of carbonates and bi-carbonates helps water PH to stabilize. The measurement unit of KH is dKH (degrees of Carbonate Hardness) or ppm, or mg/L. A high KH is good. It indicates that the tank water is safe from PH fluctuations.
  • Other chemicals – Chlorine and Chloramine (mostly when adding water), and Ammonia, Nitrates, phosphate, and Nitrites (mostly in existing tank water). These are harmful.
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Aquarium water must consist of calcium, magnesium, and iron. These are beneficial for your fishes’ health.

We add salt to the water before adding it to marine water aquariums.

We will discuss more details in the forthcoming sections.

What is Distilled Water

When water boils, it turns into steam. Water collected by condensing the steam (in a separate vessel) is known as distilled water.

Boiling water kills bacteria, releases chemicals, and undissolvable impurities remain in the original water. Thus, distilled water is purified, softest water.

Is Distilled Water Okay for Fish

It is devoid of all minerals and chemicals, whether good or bad. If you use distilled water directly for fish tanks, it will prove fatal for the fish. They will start losing the salts from their bodies, weakening their immunity.

Bettas, Goldfish, Tetras, Cichlids, Starfish – all fish species will meet the same fate. There are no exceptions.

Furthermore, it is challenging to arrange large quantities of distilled water regularly, to perform partial water changes in larger aquariums. It is also expensive. You can make smaller quantities of distilled water at home, but it is time-consuming. Home distillers come with a cost.

Thus, distilled water is not okay for fish or aquariums.

How to make Distilled Water Safe for Fishes

Distilled water is supposed to have a neutral PH (7.0). However, the atmospheric carbon dioxide starts dissolving in it, moving the PH to 5.8 and making it slightly acidic. It has zero hardness.

Below are the ways to make it safe for fishes.


Remineralization refers to adding essential minerals (carbonates, magnesium, and calcium) to distilled water, making it safe for fishes.

Use Distilled Water without Remineralization

  1. In open fish tanks, some water evaporates naturally, reducing the water level. However, the minerals in the tank water do not vanish with it. We can add distilled water directly to the aquarium to replenish the water level.
  • Some people mix distilled water with tap water to make the latter softer (or reduce hardness). They mix distilled water directly. The final water needs to meet the water parameters for the fish tank to be safe for fishes.

How to Remineralize Distilled Water

Use any of the following methods.

For freshwater aquarium:

  • Add some tap water to the distilled water.
  • Ready-to-use remineralizes are available in the market. Use as directed. It is more reliable and easy.

Distilled water for a saltwater tank gets remineralized by adding the salt mix (as it generally comes with essential minerals).

Do test the remineralized water before adding it to the fish tank.

The quest now leads to other aquarium water sources, which may be practical and more apt for regular use.

Types of Water for Fish Tanks


Well-water is predominant in rural/semi-urban areas. By default, this water is less oxygenated.

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The quality of water varies drastically from one region to another. Water from the same well might have different chemical contaminants at different periods, depending on the underground changes and other environmental factors.

It is not regulated and open to external contamination. Influences of running agricultural water (containing chemical fertilizers), industrial waste, etc., might contaminate the water from within, making it unsafe for aquariums.


Rainwater has fewer contaminants in it. Not all regions receive abundant rain. Once you collect it, atmospheric gasses will start dissolving in it. It is impossible to store rainwater for annual water changes.

You can use it as and when feasible after fixing the water quality for your pet fishes.

Spring Water

Mineral content may vary drastically from one spring to another.

It is an unregulated, natural water source. It is not uncommon for bottled and mineral water companies to use spring water after treating it.

If you have access, use it after testing and adjusting it to suit your aquarium requirements.

Tap Water

It is one of the most affordable and commonly available water sources. Many ask if we can use tap water for fish tanks.

Local authorities (like municipal corporations or local water suppliers under public water systems) regulate tap water quality. They usually add chlorine, chloramines, or both to disinfect the tap water. Both these chemicals are toxic for fish. They also kill all bacteria in the water (whether good or bad), disturbing the aquarium ecosystem.

You can get details of the chemicals in your tap water from the relevant authority. Use any of the below options to dechlorinate water.

  • Boil tap water for thirty minutes. Allow it to cool before adding it to the aquarium.
  • Alternatively, leave a bucket of tap water in the sunlight for a day. Keep stirring it periodically. The chlorine in the water will evaporate. The drawback is that it may add other unknown contaminants to open water.
  • Both of the above options do not remove chloramine.
  • To get rid of chloramine, use a liquid dechlorinator from the market. Choose the one which removes both – chlorine and chloramine.
  • Add vitamin C (acid or tablet form) to the aquarium water to remove both chemicals.
  • An activated carbon filter in the aquarium de-chlorinated water completely and removes organic compounds. However, it does not remove ammonia, nitrates, nitrites, sodium, fluoride, etc.

Thus, tap water is safe to use for fish tanks after treatment.

Mineral Water

It contains acceptable Total Dissolved Salts/Solids (TDS) ppm of water. Reminealized distilled water falls in this category.

You can add it directly to aquariums. However, being expensive, it is impractical to use mineral water bottles regularly for water changes in aquariums. The requirement would be higher for larger aquariums.

Use it sparingly to fix the aquarium water quality urgently, if need be.

Bottled Water

It is bottled water from any natural source (like a spring, river, or well) with or without any treatment/filtration.

It is not the best option for the same reasons as mineral water (refer to section 3.5). Always test and fix the water parameters before putting bottled water in your fish tank under exceptional circumstances.

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Reverse Osmosis (RO) Water

Initially, water (or any solvent) is moved through a selectively permeable membrane using osmotic pressure. This pressure causes water movement distributing all the solubles (TDS) equally on either side of the membrane.

Reverse osmosis applies a mechanical force overcoming the osmotic pressure. As a result, only the solvent (water) pushes on one side of the membrane, separating all solubles on the other side.

Depending on the cartridge quality, the resulting water will be 90 to 98 percent purer than the original water.

Certain solubles, which distilled water does not remove, are taken care of in reverse osmosis water. Some solubles may still be present in the RO water, but they are relatively low ppm and cannot harm the fish.

Unlike distilled water, it retains its neutral PH value. Besides, it is cost-effective. We can use reverse osmosis water from the home water purifier. Alternatively, we can set up a RO water purifier for the aquarium using a DIY RO kit at a very reasonable price.

Like distilled water, we need to remineralize the RO water for the aquarium or use it directly for selected purposes (refer to sections 2.2.1 to 2.2.3).

De-ionized (DI) Water

De-ionized means having no ions. This process uses electrically charged resins to capture charged ions from the water, resulting in pure water.

Bacteria and viruses are non-charged molecules in the water. Hence, DI water may still have them!

Hence, only deionized water is not the best for fish tanks, even after remineralization.

RO/DI or RODI Water

In this process, RO water is deionized, giving the purest and softest water. There are RODI water purifiers in the market. Most fish owners find RODI water essential to have a long thriving aquarium.

Since this water has no essential chemicals, we need to remineralize it before using it for aquariums. Refer to sections 2.2.1 to 2.2.3 for details.

Purified Water

We often hear the term purified water. It describes water quality in general. The water may have gone through any/several of the above treatment(s).

Check the water description label. Test the purified water parameters and adjust/remineralize suitably before utilizing it for your fish tank.

General Aquarium Water Parameters

Stable water parameters are vital to having happy and healthy fishes. Use liquid test kits or test strips, digital PH value testing meters, and TDS meters to check water parameters periodically.

 Take sample fish tank water to the local pet stores for random phosphate, copper, or oxygen level tests. They can also help find/fix the root cause.

The specific aquarium water parameters may vary according to the fish species and whether it is a freshwater aquarium or a saltwater aquarium.

However, there is a range of parameters that hold good for most aquariums. The same are listed below.

Parameter Freshwater Aquarium Saltwater Aquarium
PH Value 6.5 to 7.5 8.1 to 8.4
Temperature 72 to 82 degrees F/ 22 to 28 degrees C 73 to 84 degrees F/23 to 29 degrees C
Ammonia 0 (unless it is a new tank without fishes) 0 (unless it is a new tank without fishes)
Nitrate Less than 50 ppm Less than 1.0 ppm
Nitrite 0 ppm 0 ppm
General Hardness (GH) 4 to 12 dGH NA
Carbonate Hardness (KH) 4 to 8 dKH 8 to 12 dKH
Salinity (Specific Gravity) NA 1.025
Calcium   350 to 450 ppm
Phosphate NA Less than 0.2 ppm


There is no best water for a fish tank. Choose whatever suits you in efficiently managing your aquarium. It is about making the best of what you have.

However, RODI water is the most sought-after for its purity. It relieves you from the tension of adding any undetected minerals to the fish tank water accidentally. All you need to do is remineralize it. De-chlorinated tap water enjoys wide acceptance due to its accessibility and affordability.

Hopefully, you are now empowered to make an informed decision about choosing from various aquarium water sources. We wish you a happy fish parenting time!

About the Author

Victoria Lamb

Victoria is a freshwater aquatics specialist, fish keeper, and amphibian enthusiast. She has had more than 6 years of experience caring for aquariums and keeping several fish species, and her home boasts of 3 aquariums and a garden pond. Her goal is to educate fish owners on raising healthy and happy aquatic pets. Career Highlights:…

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