Can I use tap water for my fish tank?

Using tap water in a fish tank is a controversial topic among aquarium hobbyists. There are pros and cons to using untreated tap water versus treated water. In this comprehensive guide, we’ll go over everything you need to know about using tap water for your freshwater or saltwater aquarium fish and corals.

The Short Answer

The short answer is yes, you can use tap water for your fish tank in many cases. However, there are some important caveats. Untreated tap water often contains chlorine, chloramines, heavy metals, and other contaminants that can harm aquarium fish and invertebrates. Treating tap water with water conditioners, reverse osmosis or deionization can remove these harmful compounds and make tap water safe for aquariums.

The Long Answer

Let’s take a deeper look at the advantages and disadvantages of using tap water versus treated water for your fish tank setup.

Using Untreated Tap Water


  • Convenient and inexpensive.
  • Contains essential minerals that fish need.
  • Does not require additional equipment or chemical treatment.


  • Often contains chlorine, chloramines, copper, lead, or other contaminants.
  • Can fluctuate in pH and hardness over time.
  • May promote algae growth.
  • Can harm sensitive fish, shrimp, and coral species.

Using Tap Water Treated with Conditioners


  • Neutralizes chlorine, chloramines, heavy metals.
  • Leaves essential mineral content.
  • Buffers pH to stabilize alkalinity.
  • Readily available at pet stores.
  • Inexpensive to use.


  • Does not remove all contaminants like pharmaceuticals.
  • Can still encourage algae growth.
  • Effects are temporary and require repeat dosing.
  • Certain conditioner ingredients may be problematic.

Using Tap Water Treated with Reverse Osmosis


  • Removes chlorine, chloramines, contaminants, and impurities.
  • Discourages algae growth.
  • Ideal for sensitive species like discus.
  • Can be remineralized to desired parameters.


  • Requires installation of an RO system.
  • Removes essential minerals and buffers.
  • Waste water production.
  • Remineralization can add complexity.

Using Deionized or Distilled Water


  • Extremely pure with contaminants removed.
  • Useful for short-term transport or quarantine.
  • Can be mixed to achieve desired parameters.


  • Lacks essential mineral content.
  • Can rapidly fluctuate in pH and hardness.
  • Usually not suitable for long-term housing.
  • Fish may suffer osmotic shock.

Factors to Consider with Tap Water

There are several key factors to consider when evaluating tap water for aquarium use:

Water Source

The source of tap water and treatment methods can significantly impact its quality. Well water avoids chlorination but may have high mineral content. Surface water is more prone to contamination. Municipal water with multiple treatment stages tends to be cleaner and more consistent.

Local Water Chemistry

Tap water parameters like pH, hardness, and dissolved solids can vary drastically by region. Municipal water reports detail the local water chemistry so you can assess if it matches your fish’s requirements.

Aquarium Type

Saltwater reef tanks with sensitive corals and small marine species have lower tolerance for tap water impurities than freshwater community tanks stocked with hardy fish.

Fish/Invertebrate Species

Wild-caught fish and invertebrates adapted to soft, acidic waters are less tolerant of tap water. Fish like African cichlids and livebearers that thrive in hard, alkaline water may not require pre-treatment of tap water.

Aquarium Size

Larger tanks help buffer water chemistry changes. Frequent small water changes in nano tanks make them more prone to parameter swings if using tap water.

Filtration Capacity

Robust filtration and protein skimming helps remove some impurities from tap water over time. Heavily stocked tanks benefit from extra filtration when using tap.

Tap Water Contaminants

The main contaminants to evaluate in tap water include:


Added to kill bacteria. Toxic to fish and invertebrates even at low levels. Gas off quickly or use water conditioners to neutralize.


Longer lasting disinfectant. More stable but still toxic. Must be neutralized with conditioners.

Heavy Metals

Like copper, zinc, lead. Tap pipe corrosion major source. More prevalent in soft, acidic water. Use RO or DI to remove.

Pesticides and Pharmaceuticals

Runoff can contaminate water supplies. Difficult to remove with basic filtration. Better eliminated through chemical or RO treatment.

Making Tap Water Safe for Aquarium Use

If using tap water for your aquarium, some form of pre-treatment is usually recommended to make it safe for livestock. Here are some options:

Use a Tap Water Conditioner

Liquid water conditioners like Seachem Prime readily bind and neutralize chlorine, chloramines, metals, and some other contaminants in tap water. Dose per instructions when adding new water.

Install a Reverse Osmosis System

RO units with DI resin remove nearly all impurities from tap water. The pure RO water often needs remineralization. Can be expensive but useful for saltwater tanks.

Aerate the Water

Vigorously aerating tap water for 24 hours helps drive off chlorine gas. Provides only temporary treatment and does not remove chloramines or metals.

Use Seachem Purigen

This specialty resin removes some tap water impurities like dyes, metals, and organic compounds when run in the filter. Not a complete treatment method alone.

Blend with RO or Distilled Water

Mixing purified RO or DI water with tap water combines their benefits. Helps stabilize pH and hardness while reducing contaminants. Easier for large systems.

How to Transition Fish to Tap Water

When introducing new fish to tap water, it is best to acclimate them slowly to avoid shock. Follow these tips:

Gradually Mix in Tap Water

Initially, only replace 25% of the current water with treated tap water at water changes. Slowly increase the ratio of new tap water over several weeks.

Use Extra Water Conditioner

Double dose water conditioners when first mixing in tap water to ensure chlorine and metals are adequately neutralized.

Monitor Water Parameters

Test pH, GH, KH, ammonia, nitrates, phosphates, etc to catch any adverse changes and perform extra water changes if needed.

Reduce Feeding

Feed fish lightly while transitioning to tap water. Less food means less waste and lower nitrogen compounds as the cycle adjusts.

Quarantine Sensitive Species

House wild caught fish, scaleless fish, corals, shrimp, etc in a quarantine tank until fully transitioned to tap water chemistry.

FAQs about Tap Water in Aquariums

Can I use tap water in my saltwater reef tank?

Tap water is not recommended for most saltwater setups. The chlorine, lack of alkalinity, and heavy metals can wreak havoc on sensitive corals and small marine species. Use RO water or conditioners to remove impurities.

What’s the white cloudy stuff when I add tap water?

This is caused by a precipitation reaction when hard tap water mixes with minerals in the aquarium. It is mostly calcium carbonate and magnesium hydroxide flocs. The cloudiness should clear on its own.

Can I use hot water from the tap for water changes?

No, hot tap water often leeches more contaminants from pipes and heaters. The higher temperature also reduces oxygen saturation. Always use room temperature or slightly warm water.

Is it safe to use tap water right after a water main break?

No, water main breaks can introduce lots of sediments and contaminants. Let your taps run several minutes until water clears. Consider using bottled water for fish tanks during water advisories.

Is water conditioner necessary if my city uses chloramines?

Yes, you should still use water conditioners even if your tap water is treated with chloramines rather than free chlorine. Chloramines are very stable and will remain toxic unless neutralized.


With proper treatment and acclimation, tap water can be suitable for freshwater aquarium use in many cases. However, saltwater reef systems and tanks housing sensitive species still benefit more from using purified RO or DI water. Always research your local municipal water chemistry and take steps to remove harmful disinfectants, metals, and contaminants through chemical conditioners, RO units, or other methods. Match the fishes’ natural water parameters as closely as possible. With the right approach, you can successfully utilize tap water for water changes and top-offs as a more convenient and affordable option.

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