What Kind Of Water Is Best For A Fish Tank & Which To Avoid

Last Updated on 2023-11-09

Choosing the right water for your fish tank is crucial for the health of your aquatic pets. Understanding the available options and your fisH’s requirements is essential for maintaining a balanced ecosystem.

So, in this article, we’ll explore the different types of water sources commonly used in fish tanks and provide guidance on choosing the most suitable one for your tank!

Key Takeaways:

  • The choice of water for a fish tank significantly impacts the health and well-being of aquatic pets. Selecting the right water is vital for maintaining a balanced ecosystem in an aquarium.
  • Understanding the various water sources and their suitability for specific fish species is crucial to choose the most appropriate water type for the tank.
  • Tap water is generally a good choice for most aquariums due to its accessibility and cost-effectiveness. However, it needs to be treated with a water conditioner to remove harmful chemicals like chlorine.
  • Bottled spring water could be an alternative for smaller tanks, but it’s essential to choose one without added minerals or chemicals and still use a water conditioner to ensure safety for fish.
  • Different water sources like well water, rainwater, reverse osmosis water, and deionized water might be suitable for specific situations but may require additional treatment or mineral supplementation depending on the fish’s needs.

What Kind Of Water Is Best For A Fish Tank?

The best water for a fish tank depends on various factors, including the type of fish you have and the source of water available. 

In general, tap water is the best choice for most aquariums. It’s easily accessible, cost-effective, and generally safe for fish. However, it’s essential to treat tap water with a water conditioner to remove chlorine and other harmful chemicals before adding it to your tank.

Bottled spring water is also an option for smaller aquariums, but not all bottled water is the same. Make sure to choose spring water without added minerals or chemicals and still use a water conditioner to ensure it’s safe for your fish.

Other water sources, such as well water, rainwater, or reverse osmosis water, may be suitable for specific situations, but they may require additional treatment or mineral supplementation to meet the needs of your fish.

Remember that the best water for your fish tank depends on the specific requirements of your fish species, so it’s essential to research and understand their needs before selecting a water source. It’s also generally recommended to use water conditioner to remove heavy metals and impurities from the water.

Common Aquarium Water Sources for Fish Tank

If you’re wondering what the different types of water you can add to your tank are, then here are the most common ones, as well as why I would or wouldn’t use them!

Water SourceSuitability for Fish TanksAdditional Treatment Required
Tap WaterSuitable for most tanksWater conditioner needed
Bottled Spring WaterSuitable for small tanksWater conditioner needed
Well WaterVariesTesting and possible treatment needed
RainwaterVariesTesting and possible treatment needed
Distilled WaterNot RecommendedLacks essential minerals
Lake and River WaterNot RecommendedRisk of diseases and parasites
Reverse Osmosis WaterSuitable but lacks mineralsRemineralization needed
Deionized WaterSpecialized setups onlyRemineralization needed

Tap Water

Tap water is a common choice for aquariums. However, it often contains chlorine, chloramines, heavy metals, and other impurities that can harm fish. To make tap water suitable for aquarium use, it’s essential to use a water conditioner to remove them before adding it to your tank.

Distilled Water

Distilled water, produced by vaporizing and condensing water to remove impurities, is generally not recommended for aquariums. It lacks essential minerals and trace elements that fish and plants need. Additionally, it can cause pH fluctuations and osmotic stress in fish, making it an unsuitable choice.


Rainwater can be used for aquariums if collected and filtered properly. However, it may naturally be soft and acidic. To use rainwater in your aquarium, you should test and adjust its parameters to ensure they meet the specific needs of your fish and plants.

Well Water

Well water can vary a lot in quality, so it’s important to have it tested before using it for fish. The pH, hardness, and any contaminants in the water can affect your fish’s health. Some well water may be fine for fish, while others may need to be treated or mixed with other water sources. 

If you’re not sure if your well water is safe for fish, it’s always best to err on the side of caution and have it tested.

Bottled Spring Water

Bottled spring water, sourced from natural springs and sold in bottles, can be suitable for aquarium use. However, it’s essential to check the label for any added minerals or chemicals that might affect your aquarium’s water parameters. Cost considerations may also come into play if you have a large tank.

Lake And River Water

Using water directly from natural sources like lakes or rivers can introduce diseases, parasites, and other contaminants to your aquarium. As such, it’s generally not recommended unless you have access to a reliable and well-maintained source.

Reverse Osmosis Water

RO water, treated through the reverse osmosis process to remove impurities, is highly pure and can be an excellent choice for aquariums. However, it lacks minerals, so you may need to remineralize it before use, depending on your fish’s requirements.

Deionized Water

DI water is exceptionally pure, treated to remove ions. It’s often used in research or specialized setups where precise control of water quality is necessary. However, DI water may need to be used in combination with other water sources or remineralized to be suitable for aquarium use, as it lacks essential minerals.

What Are The Best Water Parameters For Most Fish?

Now let’s dive into the ideal water parameters that most fish, regardless of their specific species, generally prefer. As a very general rule of thumb, here are the key parameters to keep in mind:

ParameterIdeal RangeNotes
Temperature75 to 82°F (Tropical)Varies by species
pH Level6.5 to 7.5Varies by species
Ammonia and Nitrite0 ppmToxic substances
NitrateBelow 40 ppmLess toxic but still harmful
HardnessVariesGH and KH levels
Oxygen LevelsAdequateEnsure proper aeration


Most tropical fish thrive in water that’s comfortably warm, typically in the range of 75 to 82 degrees Fahrenheit. However, some coldwater fish prefer temperatures around 60 to 70 degrees Fahrenheit. It’s essential to research your specific fish to determine their temperature needs.

pH Level

A slightly acidic to neutral pH level, typically between 6.5 and 7.5, suits the majority of freshwater fish. However, certain species, like African Cichlids, may prefer more alkaline conditions.

(If you’re interested in your fish’s specific needs, then you should check out Tropical Fish A-Z. This article goes over 345 of the most common fish for your tank, and the water parameters they need.)

Ammonia and Nitrite

These toxic substances can be deadly to fish, so it’s crucial to keep their levels at zero. Regular water testing and the establishment of a well-functioning biological filter system are essential to maintain low or non-existent levels of ammonia and nitrite.


Nitrate is less toxic than ammonia and nitrite but can still be harmful in high concentrations. Aim to keep nitrate levels below 40 ppm (parts per million). Regular water changes and efficient filtration can help achieve this.


The water hardness, measured as General Hardness (GH) and Carbonate Hardness (KH), should be suitable for your specific fish. Most fish can adapt to a wide range of hardness levels, but certain species, like African Rift Lake Cichlids, have specific preferences for harder water.

Oxygen Levels

Adequate oxygen levels are essential for all fish. Ensure your aquarium has proper aeration, surface agitation, and good water circulation to maintain oxygen saturation.


For freshwater fish, maintaining a low salinity level is essential, going so far as to have no salt in the water. However, for saltwater fish, salinity should match their natural marine environment.

Chlorine and Chloramines

If you’re using tap water, you must remove chlorine and chloramines, which are often present in tap water and can harm fish. You can do this by using a water conditioner or allowing tap water to sit for at least 24 hours before adding it to the aquarium.

What Water Conditioner Should You Use For Tap Water?

If you’re wondering what water conditioner is best for your aquarium, then I’d recommend Seachem Prime. Seachem Prime is great because not only does it remove chlorine and chloramines from the tank, but it also detoxifies ammonia as well!

It’s also been voted the best water conditioner by over 170 fish keepers!

Seachem Prime Fresh and Saltwater Conditioner – Chemical Remover and Detoxifier 500 ml
  • POWERFUL TREATMENT: Seachem Prime is a complete and concentrated conditioner for both freshwater and saltwater fish tanks, working hard to remove chlorine and chloramine.

How The Nitrogen Cycle Affects Water Quality

The nitrogen cycle is a critical biological process that occurs in aquariums and aquatic environments, and it has a profound impact on water quality.

Understanding the nitrogen cycle is essential for maintaining a healthy and stable aquatic ecosystem. Here’s how the nitrogen cycle affects water quality in aquariums:

Ammonia Production:

The nitrogen cycle begins with the introduction of organic matter into the aquarium, such as fish waste, uneaten food, and decaying plant material. These organic materials contain nitrogen in the form of proteins and other compounds.

Bacteria, primarily Nitrosomonas, break down these organic materials, converting them into ammonia (NH3) as a waste product. Ammonia is highly toxic to fish and can rapidly degrade water quality.

Ammonia to Nitrite Conversion:

In the next phase of the nitrogen cycle, a different group of bacteria, known as Nitrobacter and Nitrospira, convert ammonia (NH3) into nitrite (NO2-). While nitrite is less toxic than ammonia, it can still be harmful to fish, especially in elevated concentrations.

Nitrite to Nitrate Conversion:

Another group of bacteria, including Nitrobacter and Nitrospira, further transform nitrite (NO2-) into nitrate (NO3-). Nitrate is much less toxic to fish than ammonia and nitrite, but it can still be harmful in high concentrations.

Nitrate Accumulation:

Nitrate (NO3-) is the end product of the nitrogen cycle and accumulates in the aquarium water over time. Unlike ammonia and nitrite, nitrate is relatively non-toxic to fish when present at moderate levels. However, excessively high nitrate levels can lead to health issues in fish and promote the growth of algae.

Water Changes and Nitrate Control:

To maintain water quality in an aquarium, regular partial water changes are necessary. Water changes help remove excess nitrates and replenish essential minerals and trace elements for fish and plants.

Nitrate control is crucial to prevent water quality problems. Aquarists often employ various methods, such as aquatic plants, efficient filtration, and reducing overfeeding, to keep nitrate levels in check.

Establishing Biological Filtration:

Beneficial bacteria that drive the nitrogen cycle establish themselves in the filter media and other surfaces within the aquarium. It’s essential to ensure that the aquarium’s biological filtration system is well-established and functioning effectively to maintain water quality.

Monitoring and Testing:

Regularly testing water parameters, including ammonia, nitrite, nitrate, and pH, is essential to gauge the health of the nitrogen cycle and overall water quality. Any sudden spikes in ammonia or nitrite levels can indicate problems with the cycle and require immediate attention.

Now, check out this video by Big Al’s on whether tap water is safe for your fish!

Common Chemicals And Heavy Metals That Affect Fish Health

However, it’s not just the nitrogen cycle that will affect your fish’s health either. You also need to be aware of the following:

ChemicalEffect on FishTreatment
ChlorineToxic; causes gill damageUse water conditioner
ChloramineMore toxic than chlorineSpecialized water conditioner
CopperToxic; affects nervous systemAvoid exposure
PhosphatesLeads to algae growthControl nutrient levels


Chlorine is a powerful disinfectant commonly used in municipal water treatment to kill harmful bacteria. However, it’s toxic to fish and can cause gill damage and respiratory distress. To remove chlorine from tap water before adding it to an aquarium, aquarists use water conditioners.


Chloramine is a compound formed by combining chlorine and ammonia. It’s used as a disinfectant in some municipal water supplies. Chloramine is even more persistent and harmful to fish than chlorine. 

Specialized water conditioners that neutralize both chlorine and chloramine are necessary to make tap water safe for aquarium use.


Copper is a heavy metal that can leach into aquarium water from various sources, including plumbing, decorations, and certain fish medications. Copper is toxic to fish, particularly to invertebrates and some species of fish that are sensitive to it. 

Elevated copper levels can cause damage to fish gills and nervous system. Care should be taken to avoid copper exposure in aquariums not specifically intended for fish that require copper treatments.


Phosphates are nutrients that can be present in aquarium water due to various sources, including tap water, fish waste, and overfeeding. High phosphate levels can lead to excessive algae growth, negatively impacting water quality and aesthetics. 

Algae blooms can reduce oxygen levels at night and create unhealthy conditions for fish and other aquatic life.

How Long Does It Take For Tap Water To Be Safe For Fish?

If your tap water has chlorine, you can simply let it sit for a few days to allow the chlorine to evaporate. To speed up the process, you can use an air stone to aerate the water for 12-24 hours, or boil the water for 15-20 minutes.

Does Boiling Water Remove Chlorine?

Boiling water can effectively remove chlorine from tap water. Chlorine is highly volatile in water, and when you boil it for about 15 minutes, the chlorine will evaporate. This method is useful for treating water for a small aquarium if you don’t have access to a water conditioner.

Are Water Changes Stressful For Fish?

Water changes are important for good aquarium maintenance, but sudden changes in temperature or chemistry can stress fish. To minimize stress, match the new water’s temperature and parameters to the existing water and use a gentle method to remove water.

Spring Or Distilled Water For Fish Tank?

Spring water is always the better choice over distilled water for fish tanks. Distilled water has had all of its minerals removed, which can lead to malnutrition in fish over time.


Aquarium keeping requires careful consideration of water source to ensure the comfort, health, and longevity of your fish. By tailoring your water to their specific needs, you’ll create a thriving and visually stunning aquatic ecosystem that brings joy to both you and your aquatic residents.


Ultimate Betta Fish Care Guide
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