Choosing the Best Water for Freshwater Fish Tank

Providing your fish with high-quality water is one of the most important things you can do as an aquarium owner. The type of water you use to fill and maintain your freshwater tank can make or break your fish’s health and happiness. But with so many options like tap water, bottled water, distilled water, and reverse osmosis water, how do you know which is best?

In this aquarium water guide, we’ll walk you through the key factors to consider when selecting water for your freshwater tank. You’ll learn the basics of water chemistry, the pros and cons of different water sources, and easy steps for preparing and maintaining healthy tank water. Let’s dive in!

The Basics of Water Chemistry

Before deciding on a water source, it helps to understand some basic water chemistry. The three main parameters to consider are pH, Hardness, and impurities.

pH is a measure of how acidic or alkaline the water is. It ranges from 0-14, with 7 being neutral. Most freshwater fish thrive in slightly acidic conditions with a pH of 6.5-7.5.

Hardness refers to the amount of dissolved minerals like calcium and magnesium. Soft water is low in these ions, while hard water is high. Most community tanks do best in moderately hard water.

Impurities are unwanted contaminants like heavy metals, chlorine, chloramines, and phosphates. Tap water in particular can contain high levels, so treating it is so important.

Testing your water source for pH, Hardness, and impurities is necessary. This allows you to address any issues through treatment before adding the water to your tank.

Tap Water: The Good, The Bad, and The Ugly

For its convenience and low cost, tap water is a popular choice for freshwater tanks. But is tap safe for your fish? Here’s a closer look at the pros and cons.

The Good

  • Readily available from any faucet or tap
  • Inexpensive compared to bottled or filtered water
  • Usually contains beneficial minerals that provide Hardness

The Bad

  • Can have unsuitable pH and Hardness for certain fish
  • Often contains elevated levels of heavy metals like copper
  • May have excess chlorine, chloramines, phosphates, and other contaminants

The Ugly

  • Tap water quality varies day to day based on municipal treatment
  • Some regions have dangerously high levels of contaminants
  • Old pipes and plumbing can leach metals and chemicals into the water

The verdict? Tap water is only recommended if you have knowledge of your local municipal supply and can properly treat it as needed through conditioning, dechlorination, and filtering. Avoid tap water from private or well sources due to potential contaminants.

Bottled Water: A Viable Option?

With so many people drinking bottled water nowadays, it’s understandable to wonder if it’s suitable for fish tanks too. Let’s explore the key considerations.

The Good

  • Widely available in stores and supermarkets
  • Typically free of heavy metals and chlorine
  • pH and mineral levels may be ideal for certain fish species

The Bad

  • More expensive than tap or filtered water
  • Lack of minerals can make water too soft and unsuitable for some fish
  • Some bottled waters have added minerals or pH adjusters

The Ugly

  • Testing and comparing mineral content between batches can be difficult
  • There are environmental concerns around plastic bottle waste
  • You may need to reconstitute minerals through additives

The consensus is that high-quality spring water is a safer bottled water for freshwater aquariums. Just be sure to research the brand’s water chemistry profile before purchasing. Reverse osmosis water can be used if remineralized.

Distilled vs. Drinking Water

You’ll encounter two common types of bottled water: distilled and purified drinking water. How do they compare for use in your freshwater tank?

Distilled Water

  • Water that has been boiled and condensed to remove impurities
  • Lacks beneficial minerals needed by fish
  • It has a near-neutral pH around 7

Purified Drinking Water

  • Treated by filtration, UV, or ozone to remove contaminants
  • Retains some natural minerals from the water source
  • pH varies based on the water chemistry

For Fish Health:

Distilled water is likely too pure to be used on its own. The lack of minerals can cause osmotic stress for fish. Drinking water is a better pick as it retains some hardness.

For Ease of Use:

Distilled water requires remineralizing before use, which is an added step. Drinking water may only need dechlorinating.

The Verdict:

While drinking water is the safer choice overall, distilled can work with proper remineralization. For ease of use, purified drinking water is your best-bottled bet.

The Role of Reverse Osmosis (RO) Water

Reverse osmosis (RO) filtration is commonly used to produce pure water for drinking and aquariums. But is it a necessity for freshwater tanks? Let’s break it down.

How RO Works

  • Water is forced through a semipermeable membrane to remove impurities
  • The membrane has microscopic pores that block contaminants
  • Creates water with 0 TDS (total dissolved solids)

Benefits of RO Water

  • Removes pollutants and chemicals found in tap water
  • Allows complete customization of water parameters through remineralization
  • May be essential if tap water quality is extremely poor

Potential Drawbacks

  • RO process strips water of essential minerals needed by fish
  • Remineralizing adds steps and cost for aquarium use
  • Wastes water – up to 90% of water gets discarded

Do You Need RO?

High-quality tap or spring water works well with standard dechlorinating and conditioning for most freshwater tanks. RO is generally only required in areas with heavy water contamination.

Making Distilled Water Safe for Fish

Proper remineralizing will be crucial if you use distilled or RO water before adding the water to your tank. Here are some tips to make it safe for your fish:

  • Test water’s TDS and add minerals like Seachem Equilibrium or RO Right to achieve the desired Hardness. Shoot for 4-8 dGH.
  • Raise pH if needed with buffers like Seachem Alkaline Buffer or sodium bicarbonate (baking soda). 7.0-7.2 pH is ideal.
  • Add calcium supplements like Seachem Discus Buffer to provide electrolytes.
  • Use vitamin supplements with electrolytes and slime coat builders.
  • Mix some dechlorinated tap water to provide bioavailable minerals. Start with a 25% mix.
  • Slowly acclimate fish when switching to prevent osmotic shock. Observe fish closely.

You can safely use distilled or RO water in a freshwater system with the proper blend of supplements and gradual acclimation. Test often to maintain proper levels.

Preparing Water for Your Fish Tank

Once you’ve selected a suitable water source, preparing it for your freshwater tank involves a few simple steps:

Step 1 – Dechlorinate

Use a dechlorinating product like Seachem Prime to neutralize chlorine, chloramines, and heavy metals. This removes the biggest potential toxins.

Step 2 – Adjust Hardness & pH

Test hardness and pH. If needed, increase Hardness with Equilibrium or buffers. Adjust pH with Alkaline Buffer or muriatic acid.

Step 3 – Match Temperature

Slowly adjust source water temperature to match tank water, within 2-3°F. This prevents shocks from water changes.

Step 4 – Check Ammonia & Chlorine

Confirm that levels test at zero before adding the prepared water to your tank. Then you’re good to go!

Properly treated water will provide a safe, healthy environment for your fish. Always avoid pouring untreated tap water directly into your aquarium.

Keeping Your Fish Tank Water Healthy

Maintaining high water quality in your freshwater tank involves regularly replacing some water with freshly treated water. Here are some tips:

  • Change 15-25% of the water weekly, or more often for heavily stocked tanks
  • Use gravel vacuums to remove debris during water changes
  • Test ammonia, nitrite, nitrate and pH weekly and adjust as needed
  • Use activated carbon in the filter to remove dissolved organics
  • Clean filters monthly to maximize beneficial bacteria
  • Add live plants like java fern and anubias to absorb nitrates

The keys are to limit nitrogenous waste from fish, uneaten food, and other organics while replenishing essential minerals. This provides your fish with clean, healthy water.

Purified Water: Is It Necessary?

You may wonder if purified water is mandatory for a healthy freshwater aquarium. Let’s take a look:

What is Purified Water

  • Purified water is tap water undergoing additional filtration processes to remove impurities. This includes systems like reverse osmosis.

Potential Benefits

  • Removes heavy metals, chemicals, medications that may be present in tap water
  • Allows complete control over mineral levels and water parameters
  • Can be optimized for certain fish that need precise water conditions

Downsides of Purified Water

  • Lacks essential minerals and electrolytes needed by fish
  • Remineralizing adds complexity and cost
  • Wastes water if using reverse osmosis filtration

Is it Necessary for Freshwater Tanks?

  • For most community tanks, purified water is likely not needed
  • Tap or spring water works well when properly dechlorinated and conditioned
  • Purified water best serves breeding tanks, wild-caught fish, or tanks with sick fish

The Verdict

Purified water can be useful but is not mandatory for the average freshwater aquarium. Focus instead on treating tap or spring water for chlorine, metals, and adjusting Hardness and pH as needed. This will provide healthy water without the hassle and waste of advanced purification.

What Kind of Water Should You Fill Your Fish Tank With?

As a quick summary, here are some general freshwater fish tank water guidelines:

  • Bettas – Soft, acidic water around 6.5 pH. Use RO water or rainwater.
  • Tetras/Rasboras – Lightly acidic, soft water. Purified drinking water works well.
  • Livebearers – Moderately hard alkaline water, 7.5-8.0 pH. Treated tap water is fine.
  • Cichlids – Hard, alkaline water, pH 7.8-8.4. Use tap water and buffers.
  • Goldfish – Moderately hard water, near neutral pH. Dechlorinated tap water.
  • Planted Tanks – Soft to moderately hard water depending on plants. RO water with remineralization.

Most community tanks generally thrive in moderately hard water within a neutral 7.0-7.5 pH range. Focus on properly treating your water source with dechlorinator, buffers, and supplements as needed.


Providing high-quality water tailored to your fish’s needs is crucial for their health and well-being in any freshwater tank. While tap water works well for many aquariums, also consider spring, drinking, or distilled water sources. Treat all new water with dechlorinating and conditioning supplements before use. Test and adjust Hardness, pH, ammonia, nitrates, and temperature to give your fish the best water possible. With some preparation and regular partial water changes, you can maintain clean, safe, and mineral-rich water for your freshwater buddies to thrive in.