How to Pretreat Water for An Aquarium

Balance is the way of the natural world. The sheer size of naturally-formed lakes and rivers regulates the composition of chemicals and bacteria in the water, keeping it safe for fish and aquatic life.

In nature, refuse from decayed matter, fish, and animal waste are naturally filtered, with water-based flora and bacteria removing traces of ammonia and nitrates from large water bodies.

The larger the water body, the less likely it is to contain substances that are lethal to its inhabitants.

In an aquarium, however, the situation can be a little more complex.

At Aquarium Sphere, we strive to educate and inspire the aquarists of tomorrow towards a healthier tank.

Today, we discuss the subject of pre-treating water for your fish tank, and how to ensure it is at optimum levels.

Tap Water: What You Need to Know

Most freshwater aquarium owners use tap water in their tanks due to its ready availability and cost-effectiveness.

Tap water is supplied keeping the needs of humans in mind and is, therefore, required to meet strict standards that ensure it is safe for consumption.

However, certain additives used to purify the water for consumption— including chlorine-based disinfectants and pH-raising additives— can adversely interfere with the biological cycles of fish.

Another potential danger to marine life in a fish tank is the heavy metal content of tap water. This includes cadmium, copper, lead, and zinc. 

While beneficial to fish in small doses, left unchecked, these metals could pose a threat to the welfare of your tank and prove harmful to fish.

While the chemical composition of water taken straight from the tap is the primary source of concern, another factor to bear in mind is the variability of these properties over time.

Often, in the summer months when water reserves dry out, certain districts are forced to borrow water from neighboring districts. In other instances, changes in weather patterns and industrial runoff may affect the chemical integrity of your tap water.

This leads to variability in terms of water hardness, bacterial composition, and heavy metal content of the water.

So, how does one ensure that their tap water is safe for use in an aquarium?

fish in aquarium

Choosing The Right Conditioner For Your Tank

Pre-treating water is a sure-fire method to avoiding unseemly problems with your home aquarium.

Just as water utility companies pre-treat tap water for safe human consumption, so too must we ensure that the same tap water is made healthy and safe for marine life to live in. This is achieved by treating tap water with a water conditioner.

Water conditioners are essential to your aquarium hobby kit and are designed to remove harmful chemicals like chlorine, chloramine, and ammonia from tap water.

Aqueon Aquarium Tap Water Conditioner, 16-Ounce

These can be divided into three categories, based on their use. These include:

  • Chlorine Removers: These conditioners target only chlorine. Examples include ChlorGuard by Seachem and Tap Safe by Interpet Gold
  • Chlorine and Chloramine Removers: Conditioners that remove chlorine, and are advertised to neutralize or remove chloramine. These tend to leave behind the ammonia resulting from the breaking down of chloramine. Examples include Aqua Plus by Nutrafin and NovAqua by Kordon
  • Chlorine, Chloramine, and Ammonia Removers: Conditioners that remove chlorine and chloramine, and neutralize raised ammonia levels resulting from the breaking down of chloramine. Examples include Prime by Seachem and Start Right by Jungle Lab

It is vital to find out which purifiers your local water utility uses so that you might effectively eradicate them from your aquarium or tank water by picking the type that is best suited for your needs.

If your water is treated with chlorine, any type will work for you.

However, if your water is treated with chloramine, you need the second type.

Some manufacturers who produce only the first type of conditioners omit the fact that users might require a secondary agent to treat the ammonia released from the breaking down of chloramine, with disastrous results.

Pretreating Aquarium Water for Freshwater Aquariums

Once you have identified the additives that must be removed from your tap water, and have selected the right conditioner for the job, it is time to begin treatment.

Luckily, the process is fairly uncomplicated and can be completed in a few simple steps.

  • Step One: Wash your hands and sanitize your workstation
  • Step Two: Fill tap water into a bucket or large container
  • Step Three: Add in your water conditioner in exact product-to-gallon ratio as per the instructions listed on your choice of product
  • Step Four: Using your hand, stir the water to activate the solution and ensure that it spreads through all of the water in your container.
  • Step Five: Allow your treated water to sit overnight, covered with a lid, and store in a cool dark place, before putting it in your tank.

While some recommend adding new water immediately to your tank, it is better to allow the water to sit for a while. This ensures that it matches the water temperature already in your tank and that the chemicals have taken full effect.

It is recommended to perform water changes of 25-30% every week, to avoid adverse issues like an unfavorable imbalance of nutrients and minerals, which could lead to increased algae growth and aquatic illnesses.

Be sure to test for the ideal parameters of chemicals and nutrients before performing a water change, or adding new water to aquariums.

Ideal Parameters For Water In A Freshwater Aquarium

To find out exactly what your local water contains, the quickest solution is to raise the issue with someone who is paid to know the answer.

This includes fish stores that reside in the same water district as you, or the local water utility themselves. The EPA requires utilities to provide a periodic water report. This report is in the public domain and should be relatively easy to secure.

However, it is recommended to purchase kits to measure and test the exact levels of your aquarium water, both before, as well as post-treatment to ensure the levels are stable.

It is beneficial to repeat these tests every few weeks.

fish in an fish tank

Here’s a helpful list of the various parameters to keep in mind.

pH Levels

The pH level of your tank water indicates the acidic or basic quality of your solution.

While most fish can survive in a wide range of pH levels, it could make matters difficult if the measure is extremely high or low.

The normal pH range is 6.5 to 8.2

Chlorine and Chloramine

Toxic to fish, these are added to tap water to make it safe for human consumption.

Tap water must be purified using a water conditioner to remove any chlorine or chloramine that may be present.

The ideal results of a chlorine and chloramine test in tank water should be 0.0 mg/L.

Ammonia (NH3)

Ammonia is toxic to fish at concentrated levels. Naturally excreted through the gills, ammonia is converted into nitrite by the so-called “good” bacteria, introducing nitrogen to a tank’s water composition as part of the natural nitrogen cycle.

The ideal results of an ammonia test in aquarium water should be 0.0 ppm (mg/L)

Nitrite (N02)

Caused by the decomposition of non-fish waste, including leftover, uneaten fish food, as well as the natural oxidation of ammonia by healthy bacteria, nitrite is toxic to fish.

The ideal results of nitrite tests in tank water should be 0.0 ppm (mg/L).

Nitrate (N03)

While lower in toxicity than nitrite, high concentrations of nitrate over extended periods may have harmful consequences for fish. Nitrite is produced by the biological process of nitrite oxidation by “good” bacteria.

The normal range for Nitrate in aquarium water is between 0 to 30 mg/L.

General Hardness (GH)

GH indicates the measure of magnesium and calcium ions in water. Water with a low GH level is called soft water, while a high GH indicates hard water.

While most fish can survive in a wide range of GH, particular species are more sensitive to GH change.

The normal GH range for a fish tank is between 100 to 250 mg/L.

Carbonate Hardness (KH)

KH is the measure of carbonates and bicarbonates in your aquarium water. When at optimal levels, KH aids in stabilizing aquarium pH levels.

To keep KH in check, keep up a healthy habit of periodic and partial water changes.

The normal KH range for aquariums is between 120 to 300 mg/L.

Also read: How to Lower KH (Carbonate Hardness) in an Aquarium?


Temperatures that are too hot or too cool can cause serious harm to your fish tank.

It is advised to use an aquarium heater to regulate underwater temperature, and retain consistency with every water change.

The normal temperature range for tanks is between 74 to 82 degrees Fahrenheit. (23 to 28 degree C).

Looking for more helpful tips and tricks to caring for your aquarium? Head over to our website for valuable resources on aquarium care, and join our ever-growing community of dedicated aquarists today!

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