Fish Tank Water – The 9 Best Sources For Your Aquarium

The most important thing about your aquarium is water! Without water, there’s no aquatic life… no decorations – nothing!

But, can you use any source of water for your fish? The answer is subjective!

In this article, I will list the 9 best fish tank water sources for your freshwater and saltwater fish tanks.

Let’s go!

Key Takeaways

  • Tap water is the most available source of aquarium water for most tropical or freshwater aquariums.
  • Distilled water is actually a great source of aquarium water only if it is remineralized for aquarium use. 
  • RO water is a big no-no for saltwater fish tanks. Go for RODI water systems for a saltwater aquarium and use a quality saltwater mix for the best water you can provide to your marine animals.
  • Never use 0 TDS water directly in an aquarium unless for top off purposes. Always remineralize.

What Is TDS?

TDS stands for Total dissolved solids which represents the total concentration of dissolved solids in water, such as inorganic salts, and organic matter. The total TDS level in the aquarium setting determines the amount of dissolved solids present in the aquarium water.

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Fish, in general, prefer a stable environment with the same levels of TDS and pH as their natural habitat. Many freshwater aquarium fish prefer a TDS of around 100 – 400. Too high TDS concentrations would cause excessive algae blooms or death of your fish at extreme levels. Too low levels will adversely affect the health of your fish and would kill them if you use 0 TDS water. Note that some fish do better with high TDS and others prefer lower TDS water. Inverts prefer the lowest TDS and discus fish are among one of the tropical fish species that demand a low TDS water source.

The Best Fish Tank Water Sources To Consider

Here are the 9 best fish aquarium water sources you should consider for your adorable pets. We have a video from our YouTube Channel that you can follow along. Be sure to subscribe if you enjoy our content as we post new videos every week.

1. Municipal Tap


Many aquarists use municipal water supply or municipal tap water for their fish, not knowing the fact that it could be extremely dangerous. However, research suggests otherwise!

According to the Central Institute of Fisheries Association1:

Tap water is probably the safest source of aquarium water for the majority of tropical fish. However, pH, dissolved oxygen, hardness, ammonia, chlorine, temperature, salinity, etc. play an important role in the management of an aquarium.

As mentioned, dissolved oxygen, pH, ammonia, chlorine, chloramine, nitrates, silicates, and high phosphate trace elements may alter the quality of your tap water. Therefore, regular filtration systems are needed to improve aquarium water quality. Hardness is also a factor. I’m supplying a map of carbonate hardness by region in the US for your reference (source):

Pros of tap water

  • It is a readily available and super cheap option
  • Contains many essential minerals such as calcium, and magnesium, which are important and safe for fish
  • Mostly consistent in terms of water parameters like pH levels, water hardness, etc. However, the water hardness depends on the area which may be high in some regions, low in many regions, and ideal in some.

Cons of tap water

  • It has trace elements of Chlorine and Chloramine that are toxic to fish and may damage their gills
  • Tap water may contain heavy metals, industrial chemicals, and pesticides that may adversely affect your fish.
  • In many parts, the pH levels, water hardness, and other water parameters are always fluctuating, leading to unnecessary stress in fish.

How to make municipal tap water safe for fish?

Since tap water is the safest option for your fish, it’s a shame to give in to the cons. There are always ways to make municipal water safe for your fish.

  1. Always test the water before adding it to your aquariums for ammonia, nitrates, nitrites, and other essential parameters to make sure it does not contain harmful chemicals or heavy metals.
  2. It is recommended to neutralize Chlorine and chloramine with a de-chlorinator or water conditioner to maintain a healthy aquarium.
  3. Many studies suggest storing water in large containers before adding it to the aquarium allows Chlorine to evaporate naturally. Therefore, you can opt for this method.
  4. Regular and weekly water changes and tank maintenance are crucial to reduce the amount of contaminants and other chemicals in your aquarium.

One thing to note is that tap water is often times loaded with high phosphates or silicates, which will encourage the growth of algae. For those who are considering planted tanks or reef aquariums, it would be best to switch to the other sources we are going to mention in this article.

2. Distilled


Distilled water is devoid of all the contaminants; Chlorine, heavy metals, and other harmful chemicals that may affect your fish’s quality of life. Ideal water choice for your fish tank, no?


If you’re someone who’s always running errands and has no time to make grocery runs last minute, I’d never suggest using PURE distilled water for your fish tank.

That’s because distilled water, despite being free of contaminants, lacks many essential minerals found in natural water sources.

Therefore, to use distilled water for your fish tank, you need to first remineralize distilled water in various ways. Only then you can use it for your tank water. You will want to use a product like SeaChem Equilibrium to bring your minerals to natural levels, which will make it safe for aquarium usage. While you can use distilled water to top off, I would not recommend using pure distilled water for your water changes. Remineralization is a must!

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Pros of distilled water

  • Distilled water is free of all the impurities, and contaminants, including those pesky chlorine, and chloramine.
  • You will not have aquarium blooms due to the introduction of nutrients from your source water

Cons of distilled water

  • It lacks essential mineral content such as Calcium, Magnesium
  • Distilled water is more expensive than tap water. 
  • The water is pH neutral, which may require buffer to increase or lowering agents to decrease

How to remineralize distilled water for your fish tank

So what if distilled water lacks essential minerals? There’s a way to solve that strategically.

  1. You can use commercial remineralizes in the market that helps to remineralize your distilled water, safe for aquarium use.
  2. For easy and cost-effective remineralization, many aquarists mix tap water with distilled water to make up for those missing essential minerals. However, make sure the ratio of tap water and distilled water suits the specific requirements of your fish and aquatic plants. Using a TDS meter can help in this process.
  3. The best recommended method is to add additives like Seachem equilibrium to get your trace elements up
  4. In your filters or substrates, you can also add crushed coral, limestone, and aragonite so they gradually release calcium and magnesium into the water, increasing water hardness and pH. You can use peat moss or driftwood to lower pH and hardness

Regardless of what you add to your aquarium, it’s essential to do regular water testing of water parameters and adjust everything to keep the environment healthy for your fish. 

3. Rainwater

Rainwater System

As I said, municipal tap water is one of the safest aquarium water for your fish, but rainwater is the purest water for fish. However, like all water forms, it comes with its pros and cons and limitations.

So, let’s just discuss.

Pros of rainwater

  1. Rainwater is naturally soft and acidic and ideal water for tropical fish, including tetras, discus, and other shrimp species.
  2. It is free of chlorine and chloramines. 
  3. Rainwater eliminates the use of water softeners in many cases because it is naturally low in dissolved minerals.
  4. It contains many microorganisms essential to maintaining a healthy ecosystem in your aquarium. 

Cons of rainwater

  1. Rainwater is not always safe for fish. The safety of rainwater in your tank largely depends on your area and atmosphere, which may include acid rain, dust particles, pesticides, and other additives.
  2. Just like distilled water, the pH and water chemistry of rainwater fluctuates, maintaining a low buffering capacity, which could be detrimental to fish.

How to use rainwater for aquarium use

  1. Collect water in clean and sealed containers to avoid contamination. 
  2. Use the water instantly to avoid the chances of stagnation. 
  3. Never collect the water from the first rain as it may contain pollutants from the roof, trees, or other surfaces.
  4. Filter the rainwater with activated charcoal to remove contaminants or use a UV sterilizer to kill harmful pathogens 
  5. Test the water regularly for pH, water hardness, ammonia, nitrates, and nitrites.
  6. Add minerals and buffers to make the necessary adjustments for your fish. 

4. Well


Well water is also commonly used for aquarium water. And it’s one of the best water sources for freshwater fish tanks as it has zero chlorine or chloramines. However, I recommend using it with automated systems.

Pros of well water

  1. Zero traces of chlorine or chloramine.
  2. It has beneficial nutrients like Calcium and Magnesium, useful for fish and many aquatic plants.
  3. Well water is usually stable with consistent water parameters, including pH, hardness, etc. 

Cons of water water 

  1. Despite being free from chlorine and chloramines, well water may contain other potential contaminants and pollutants such as nitrates, nitrites, pesticides, and heavy metals. 
  2. Water from a well is usually hard water or high/low in pH, which could be stressful for your fish. 
  3. Sometimes, gases like Methane can accumulate in the well and give off a pungent smell. Therefore, aeration of well water is essential to keeping the water safe for your fish.

5. Lake And River Sources

Lake Summertime

If you’re setting up a biotope aquarium, using lake and river water is an ideal option to mimic the natural environment. 

However, like any other source of water, lake, and river water sources have their pros and cons. 

Pros of Lake or River water

  1. It provides a natural environment for your fish in a natural biotope. 
  2. Lake or river water contains microfauna and natural bacteria to establish a balanced ecosystem.
  3. No traces of chlorine or chloramines.

Cons of the lake or River water

  1. May contain harmful bacteria, viruses, or parasites.
  2. It could be polluted depending on the location.
  3. May introduce many invasive species such as snails, pests, or plants into your aquarium.
  4. River water has fluctuating parameters. Therefore, water testing is essential before adding lake or river water into your home aquariums.

6. Bottled Water


If you’re choosing the best source of water for yourself, why not get it for your aquarium pets? It seems like a great idea, but you have to take some precautions first. However, it’s always good to use in a pitch. If you’re serious about trying bottled water, I recommend using bottled spring water as it’s readily available in the market and an excellent option for your freshwater fish tank.

Pros of bottled water

  1. Bottled water is 100% pure water that undergoes a filtration process to remove harmful contaminants. 
  2. No traces of Chlorine or Chloramines.
  3. You get consistent water parameters, which is beneficial for maintaining a healthy ecosystem. 
  4. It’s easily available in the supermarkets.

Cons of bottled water

  1. Regular water changes in your aquarium might be very costly, especially for large fish tanks.
  2. Bottled water comes in plastic bottles that contribute to landfill waste and pollution. So, not environmentally friendly. 
  3. Filtered water lacks many essential nutrients necessary for the health of your fish. 
  4. Could be low in TDS – always check your source

7. Reverse Osmosis Water (RO)

RO Filter

If you have a heavily planted tank, you should go for reverse osmosis water. However, for saltwater tanks, Reverse osmosis is the preferred option. You can safely use reverse osmosis deionized water for saltwater tanks for top off or after you mix it with a quality marine tank salt mix. 

Pros of reverse osmosis (RO) 

  1. RO water is free of contaminants such as Chlorine, chloramine, and heavy metals.
  2. Since there are lack of nutrients in RO water, it prevents algae growth in the aquarium. 
  3. It’s more suitable for sensitive fish species such as shrimps.
  4. You can also use RO water for home usage 

Cons of reverse osmosis (RO)

  1. Since RO water goes through a purification process. The process of filtration can contribute to longer water change prep times.
  2. It is costly to set up an RO system. The maintenance also comes with additional cost. 
  3. RO water is not ideal for saltwater aquariums. 

How to remineralize RO Water for aquarium use:

If you need higher TDS than what RO water outputs, it is essential to remineralize RO water before use:

  1. You can use commercial remineralizer to add necessary minerals according to the volume of water.
  2. If you want to use natural alternatives, go for crushed coral, or aragonite to naturally introduce minerals into water. 
  3. The best way to mix RO water with tap water or well water is to reintroduce minerals.

8. Reverse Osmosis Deionized (RODI)

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If you have a saltwater fish tank, I highly recommend using reverse osmosis deionized water. It’s a combination of reverse osmosis water and deionized water process to purify water completely. Of all water options available, this is the go to for any saltwater tank owner.

Pros of RODI water

  1. Highly water quality available in the hobby
  2. It allows precise control over water parameters, ideal for sensitive tank setups.
  3. RODI system gets rid of Chlorine and Chloramine completely.
  4. There are no potential algae or microorganisms in the aquarium.
  5. An ideal water source for heavily planted tanks and delicate fish species such as Discus. 
  6. A must for reef tanks

Cons of RODI water

  1. The cost of the RODI system altogether is very high, including the replacement items and DI resins. 
  2. It requires lots of maintenance because of the complexity of systems.
  3. Requires remineralization to be used in freshwater systems
  4. pH neutral – water requires buffering for fish that prefer higher pH or hardness

9. The Ocean (For Saltwater Only)


Natural ocean water is a great option only if you have saltwater tanks or live near coastlines. The ocean water contains microfauna and natural bacteria that are very useful for saltwater fish tanks. 

Pros of ocean water:

  1. It contains a natural balance of salt minerals and other beneficial organisms completely mimicking the natural environment.
  2. It’s an affordable option for aquarists living near the coastlines. 
  3. There’s no salt mix required to maintain the salinity of the aquarium water. 

Cons of ocean water

  1. It might contain many pollutants, depending on the location. 
  2. There might be a potential risk of introducing pathogens, algae, and other bacterial infections, harmful to your fish. 
  3. The saltwater use may not be compatible with the ocean fish you keep. Check parameters!


What is the best water to put in a fish tank?

The only answer to this question is: It depends! Any water source could be good or bad depending on your location and other external factors. However, according to the Central Institute of Fisheries Association:

Tap water is probably the safest source of aquarium water for the majority of tropical fish. However, pH, dissolved oxygen, hardness, ammonia, chlorine, water temperature, salinity, etc. plays an important role in the management of aquarium. All these are described here with tips for cleaning the aquarium and selecting an aquarium filter.

Therefore, tap water, when properly filtered and clean, is the best aquarium water source.

Should I use distilled or purified water in my fish tank?

You can use distilled water in combination with tap water for fish tanks. Because distilled water has very low mineral content and tap water can be hard. Therefore, they both can neutralize each other to keep your fish healthy. Always make sure to use a decholorinator when using any solution of tap water!

Is spring water or distilled water better for a fish tank?

Yes, spring water is considered a better source of water than purified water. But avoid using spring water if it’s polluted enough or doesn’t contain natural minerals. Distilled water requires demineralization to be safe to use in aquariums. It is excellent for top off.

What kind of bottled water do you use for a fish tank?

Deionized water is an excellent choice for top off water because it’s free from contaminants and harmful substances. Spring water is best in a pitch and likely does not require demineralization. At all costs, avoid using carbonated water in your fish tanks.

Does distilled water need a conditioner for the fish tank?

Yes, distilled water needs to be remineralized to be used for aquarium water. It is essential for distilled water to undergo the heating and cooling process for the removal of impurities.

How long can fish survive in tap water?

Without proper treatment, your fish will die in a matter of a few hours. I have personally seen this happen with a fish of mine who accidentally used pure tap water when changing their water. Fish will not last long with untreated tap water! If you accidentally do this, promptly remove the fish, and place them in treated freshwater. Treat the tank with a dechlorinator than promptly change the water and add in treated water to replace it.

What kind of water do you use in a fish tank?

You can use any type of water with proper treatment and guidelines. Many freshwater aquarists use treated tap water. Planted tanks will usually use RO water. Saltwater tanks will usually use RODI or distilled water. 

Final Thoughts

Choosing aquarium water is the most crucial step to keeping your fish in a healthy environment. Therefore, choose wisely. You can use a combination of tap water and distilled water or if your tank is too small, you can even go for bottled spring water. The choice is entirely up to you, just make sure you’re following all the instructions and guidelines about water usage. If you have any questions, leave a comment below and I’ll get back to you. Thanks for reading!

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