The Best Water for Your Aquarium & Fish [In 2022]


Having a fish tank at home is a great way to relieve stress and have some fun. While fish are pretty low maintenance as pets, you do need to choose the best water for your fish tank to ensure their health and well-being.

Best Water for Your Fish Tank: Short Summary

There are several different kinds of water you can use in your fish tank. No matter which water you choose, it will need additional conditioning to make it safe for your fish. Read for a breakdown of the merits and demerits of each kind of water.

All Aquarium Water Sources

You have plenty of options while choosing water for your fish tank. If you have a freshwater tank, you can use the water directly from the source. However, if you have fish that need saltwater, you will need to include a salt mix.

Whichever kind of water you end up choosing, you will probably need to give it some kind of treatment to make it suitable for your fish. Make sure the water is safe for your fish by always testing it before you place the fish in it.

Tap Water

Tap water comes from your local municipality, which means that it has already been filtered and disinfected. It is the easiest method to obtain water for your fish tank and it is already free of bacteria and harmful pathogens.

However, you still need to be careful because not every locality gets the same quality of tap water. Sometimes it is contaminated or otherwise polluted.

The tap water in some localities has toxic levels of iron and magnesium which are not suitable for fish. It may even contain small amounts of asbestos or ammonia, which are extremely harmful to fish.

Most municipalities use chlorine in their filtration process to disinfect the water. It is very effective for killing bacteria, but it is also toxic for fish.

The chlorine in tap water will also get rid of the bacteria in your water filter that break down chemicals like ammonia and nitrite in fish waste.

Well Water

If you have access to a well, you can rest assured that the water you draw from the well does not contain chlorine. But this doesn’t mean that well water is perfectly safe and suitable for use in fish tanks.

Well water is not regulated by any authority. While it is free of disinfectants like chlorine, it can also contain any number of pathogens depending on the area you live in.

If there are any farms in your vicinity, it is possible that agricultural runoff has infiltrated your well water. This means that it could contain fertilizer elements like nitrates or coliform bacteria.

On the other hand, industrial runoff could introduce harmful organic compounds from pesticides, herbicides, dyes, solvents and industrial cleaners into your well water.

Well water also differs in hardness and pH and has high levels of oxygen, so it will need conditioning and aeration to make it safe for fish.

Bottled Water

Bottled water is filtered and passes a safety test before it reaches you, so it must be safe for fish too, right?

Well, not really. Bottled water originates from spring water or well water or is filtered water. No matter the source, it goes through a basic filtration process that removes pathogens to make it safe for humans.

This process can remove bacteria that were beneficial for fish or it might contain some minerals that are beneficial for humans but harmful for fish. The only way to be sure it is okay for your fish is to test it and adjust accordingly.

It is also important to note that bottled water is a lot more expensive than other kinds of water. Using bottled water will make maintaining a fish tank a very expensive affair, especially if your tank is very large.


Rainwater is also a viable source of water for your fish tank. However, it comes from a natural source, which means that its pH can be different each time. You will need to test it and treat it accordingly.

Rainwater is also quite low in minerals. As it falls from the sky, it can pick up pollutants or pathogens along the way. If there is smoke or chemicals suspended in the air (very likely in large cities), the rain will absorb it as it falls.

While pollutants are a constant problem, rainwater is also not always available. Unless you live in an area with constant rainfall, you can depend on it as a source of water for your fish tank. Storing rainwater safely is also an issue.

Distilled Water

Distilled water is an extremely pure form of water. It is made by boiling water until it turns to steam, leaving behind all dissolved minerals and contaminants. The steam is then channeled to another container, where it condenses and turns back to water.

Distilled water is not as expensive as bottled water and it is easily available. However, the distilling process strips the water of all its minerals. You will need to add minerals to it before using it in your fish tank.

Distilled water is not a viable source of water for large aquariums, because you would need to buy it in extremely large amounts.

Filtered Water

Most people use a filtration method at home to make tap water safe for drinking. Reverse osmosis or RO is the method most often used for home filtration.

Water naturally moves from places with high particle concentration to places with low particle concentration.

In reverse osmosis, this natural process is turned around by passing water through a membrane that holds back particles on one side, letting clean water flow out to the other side.

This is a very effective filtration method. If you already use RO water at home, it is a good choice for using in your fish tank too. Keep in mind that you will need to remineralize the water before you add it to your fish tank.

Deionized Water

Deionization is also a filtration method, but it works differently than reverse osmosis. Deionization systems have a chemical resin that holds contaminants with an electric charge and exchanges them for hydrogen ions.

Deionized water is free of mineral and chemical elements. It even takes care of chemicals from pesticides. You can hook up a deionization unit to your aquarium to supply your fish tank with clean, pure water.

The only problem is that deionization does not get rid of bacteria or pathogens. Like all kinds of water, it also needs to be adjusted to make it suitable for fish. It needs remineralization as deionization strips away all minerals from the water.

Conditioning Water for Your Aquarium


Conditioning means that you test the water for a few key parameters and adjust it accordingly to make sure it is safe and healthy for your fish. There are the 3 main parameters you need to test:

  • pH: This is the most basic parameter of water. pH or power of hydrogen refers to the concentration of hydrogen in the water. It determines whether the water will be alkaline or acidic. A pH of 7 is considered neutral.

    However, the perfect pH level depends on the kind of fish you have. Freshwater fish do well in pH levels between 6.5 and 8.5 whereas saltwater fish might prefer a pH above 8.
  • kH: kH refers to the concentration of carbonates and bicarbonates in the water. If the kH is too low, it makes the pH unstable. Big swings in the pH level will make the water unsuitable or harmful for the fish.
  • gH: The hardness of the water is measured by the gH levels. Hardness depends on the amount of calcium and magnesium ions in the water. This is especially important for freshwater fish, while saltwater fish are mostly indifferent to gH levels.

Frequently Asked Questions

What Is the Best Water to Use In a Fish Tank?

The best water to use in a fish tank depends on the kind of fish you have and the general quality of water in your area. Tap water, RO water and deionized water are all good choices

However, every kind of water needs additional conditional before it can be used in a fish tank. You need to check the pH, gH and kH levels of the water and condition it to make it optimal for your fish tank. Remineralization is also needed for filtered water.

Is It Ok to Use Distilled Water in a Fish Tank?

Distilled water can be safely used in a fish tank. It does need to be remineralized because the distillation process strips it of essential minerals that are beneficial for fish. It is not feasible as a water source for large aquariums as you would need to buy it in huge quantities.

Parting Thoughts

Providing your fish with clean, good quality water is very important for their health and well-being. If the water is clean and free of pathogens or harmful bacteria, it is a good source of water for your fish tank.

Generally, RO water, distilled water or deionized water are good sources as they are free of bacteria. Tap water and well water need to be tested for pathogens and chlorine before using them in a fish tank.

No matter what the source is, you do need to test the water and condition it accordingly to make it suitable for your fish.

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