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  • Writer's pictureStewart Kessel

Beginners Guide to Aquarium Water Chemistry

Updated: Jun 10

Why is understanding aquarium water chemistry so important?

The chemistry of your fish tank water is fundamental to your aquarium's aquatic ecosystem and the health and well-being of your fish and other livestock. A basic understanding of the natural processes and the critical water parameters will help ensure your fish survive and thrive in their environment.

Beginners guide to aquarium water chemistry

Alternatively, just consider simply that your fish and other livestock live in the same body of water that they eat and produce waste. How suitable is it for their survival? Invisible to the eye, harmful chemicals could build up without you knowing.

"Just because the water looks crystal clear and clean does not mean it is suitable or safe for your fish."

That is why it is essential to understand the quality of your aquarium water chemistry.

What is meant by Aquarium Water Chemistry?

At a fundamental level, the chemistry of water is simple. Water consists of two hydrogen atoms and one oxygen atom, or to represent it chemically, H2O. However, the water in an aquarium, or indeed in the natural environment, is not pure. It will, for example, contain many different minerals, dissolved salts, and organic waste products. Quantifying these and other materials defines water chemistry or, to be precise, water quality. Together with other factors such as water temperature, these make up the water parameters we are interested in when keeping a freshwater aquarium.

The "Nitrogen Cycle" is nature's fundamental process that converts fish and other aquarium waste products into less harmful materials. The nitrogen cycle and regular maintenance enable you to maintain a healthy environment for your aquatic livestock. The process converts toxic ammonia and nitrite to less toxic nitrate. Measuring these and other key water parameters helps us understand this process, ensuring it remains balanced. Read our more in-depth guide to the nitrogen cycle to learn more about the process.

Learning about the nitrogen cycle is vital to understanding how to mature your aquarium's biological filter and how we can monitor this by measurement of some key water parameters.

When selecting livestock, we must also be mindful of their natural habitat and research whether, for example, they occupy hard, alkaline waterways or soft-acid rainforest streams. Tailoring and maintaining the aquarium water chemistry to the specific water conditions of their natural environment will help keep your fish and other livestock in good health. It is also essential if you want to observe the natural behaviour of your fish and are considering any breeding project.

Hopefully, we have established that a basic understanding of aquarium water chemistry is helpful and essential to maintaining a healthy aquarium and its inhabitants.

"In fact: it is often suggested that fishkeeping is more about keeping water than fish."

So let us now learn more about which water parameters are critical to the freshwater aquarium fish keeper.

7 Key Aquarium Water Parameters

When maintaining a freshwater aquarium, the following are the critical water parameters that you should monitor as part of any aquarium health check:

1: Ammonia (NH3)

Determining the presence, or more accurately, the absence, of ammonia, is crucial for the health and welfare of your aquatic livestock.

Ammonia is highly toxic to fish and other aquatic livestock. It will cause inflammation of the fish's membranes even at trace concentrations, particularly the gill membranes, causing the fish to gasp at the water's surface. It will also irritate and cause a change in the fish's surface mucus layer leading to bacterial and parasitic infections.

Typically, ammonia in an aquarium is due to fish and other livestock excretion as part of their metabolism. In addition to fish waste, the breakdown of proteins in uneaten food and the decay of dead livestock will also produce ammonia.

Depending on the acidity or alkalinity of the aquarium water, ammonia may be present as ammonium. In acid, low pH conditions, ammonia (NH3) ionisation to ammonium (NH4+) occurs. Ammonium is practically non-toxic to fish, but this ionisation process is reversible, i.e., the relatively harmless ammonium can quickly revert to toxic ammonia.

NH3 + H+ < = > NH4+

The equilibrium, how much of each is present, depends on the water's pH, temperature, and buffering capacity. Read our more in-depth guide about ammonia and ammonium [LINK].

Spikes in ammonia indicate there may have been a sudden change in your aquarium water chemistry; see our article concerning old tank syndrome. It is possible that your biofilter has crashed or has insufficient capacity for the bio-load in your aquarium. If the stocking level has not changed, this could also be due to the decay of unnoticed fish deaths or the breakdown of uneaten food from overfeeding.

There is no safe ammonia level in your freshwater aquarium, so regular maintenance, keeping within stocking guidelines and routine monitoring of ammonia is strongly advised to keep your aquarium livestock in good health.

2: Nitrite (NO2)

Nitrite produced by the breakdown of ammonia by Nitrosomonas bacteria, as part of the natural nitrogen cycle in an established aquarium, is also toxic to fish and other aquatic livestock. Although, as with ammonia, the physical observation is the same with fish gasping for air at the aquarium water surface, nitrite acts differently. Nitrite will combine with haemoglobin in the blood, which usually carries oxygen around the body, to form a compound that cannot perform the same function. Different species of fish have varying abilities to recover from nitrite poisoning.

The presence of nitrite in a newly established aquarium undergoing a fishless tank cycle is indicative that the aquarium filter is not yet fully mature. Read our guide on fishless cycling to mature an aquarium filter.

In an older established aquarium, the presence of any nitrite, typically accompanied by ammonia, suggests that the aquarium filter has 'crashed,' i.e., it can no longer convert nitrite into the less toxic nitrate.

As with ammonia, there is no safe nitrite concentration.

3: Nitrate (NO3)

Nitrate is present in most tap water supplies. In an aquarium, Nitrobacter bacteria break down nitrites to produce nitrate as the final stage in the nitrogen cycle.

"The absence of nitrate can be more worrying as it may indicate that the biological filter in your aquarium is not working."

Nitrate is much less toxic to freshwater fish and aquatic livestock, although longer-term exposure to high concentrations can affect some sensitive species, particularly growth rate and breeding behaviour. Fish eggs and fry are also more susceptible than adult fish.

A benefit of a planted aquarium is that plants use nitrate as a nutrient. This factor, combined with regular aquarium maintenance, particularly water changes, will help keep the nitrate levels in check. As a guide, keeping the nitrate levels below 40 ppm is beneficial.

4: General Hardness (GH)

Water hardness is something that most of us generally understand. Either from the limescale build-up in kettles and other household appliances if we live in hard-water areas. Or the ease at which soap will lather in soft water areas.

General hardness defines the total amount of all minerals dissolved in the aquarium water, i.e., magnesium and calcium dissolved salts and other trace minerals.

The presence of salts and other minerals in aquarium water is essential for the health of fish and other livestock. The habitat for some fish species is hard water, mineral-rich rivers, or lakes, whilst other fish species live in very soft bodies of water. Hence, understanding your water hardness values is essential when selecting suitable aquatic species for your aquarium.

5: Carbonate Hardness (KH)

Carbonate hardness is, as the name suggests, a measure of the carbonate and bicarbonate minerals in the water. Sometimes referred to as total alkalinity, it is also a measure of the aquarium water buffering capacity, a critical factor in pH stability. Learn more in our article on pH and buffering capacity.

As part of an aquarium's lifecycle, minerals in the water measured as general hardness (GH) and carbonated hardness (KH) are constantly naturally depleted. Minerals are absorbed by living organisms such as fish, invertebrates, and the bacteria in your filters. Biological processes such as the decomposition of organic waste into weak acids and the conversion of carbon dioxide into carbonic acid also consume carbonates from the water.

The depletion of minerals, particularly carbonates, is a common cause of issues in an established aquarium, often called old tank syndrome.

As intimated above, there is no single correct combination of values for General and Carbonate hardness. The suitable values depend very much on the livestock in your aquarium. However, it is essential to research and monitor the optimum values once established in your aquarium to ensure they remain stable.

A word of caution: for most home aquarium setups, a minimum carbonate hardness of at least 4°d KH at all times is advisable, as this will help maintain the aquarium's pH stability.

6: pH

The pH value is a way of recording the degree of acidity or alkalinity of your aquarium water. On a measured scale between 0 and 14, a value below pH7 is considered acidic, pH7 is neutral, and values above pH7 are alkaline (or basic).

It is important to note that pH is a logarithmic scale. That means even a small change in value is a significant variation.

Fish species are biologically coded to their natural environment and thrive best when kept in the same water conditions. You need to ask yourself whether your fish come from soft acid streams from a rainforest or hard water alkaline lakes. The pH value and other parameters, such as water hardness, are essential to mimic the natural habitat of your chosen fish species.

As mentioned above, the buffering capacity of your aquarium water will determine the stability and potential for any swings in its pH value. To learn more, read our article on buffering capacity.

Furthermore, as previously mentioned, the pH value of your aquarium water will affect other parameters, such as the ammonia and ammonium balance. Therefore, monitoring changes in pH is a good aid in diagnosing issues in your aquarium.

7: Temperature

Your freshwater aquarium may be Coldwater, Temperate or Tropical. Fish are poikilothermic, which means the surrounding water controls their body temperature and metabolism. Therefore, you must maintain and monitor the aquarium's water temperature to match your livestock's natural requirements. In all cases, there must not be any sudden or prolonged changes in the measured temperature.

Temperature also affects oxygen levels and other chemical balances. Higher temperatures cause the dissolved oxygen level to reduce, something to consider when warmer weather adversely influences your aquarium water temperature. See our article on how to cool and aqaurium during hot weather.

Other Water Parameters and tests

In addition to the above key water parameters, other tests are applicable for specific situations. Additional parameters could include:


Tap water is typically treated with chlorine (Cl) to make it safe for human consumption. However, this is deadly to aquatic life, and hence tap water must be pre-treated to eliminate chlorine before use in your aquarium. Test kits are available to determine the presence of chlorine. Still, in the opinion of most aquarists, this is unnecessary if you have used a proprietary de-chlorinator to condition your aquarium water.

Phosphate (PO4)

Phosphates are an essential nutrient for both plants and animals. Therefore, generally, they are deemed to be safe. However, excessive phosphate levels can lead to unwanted plant growth, such as algae blooms. These blooms in the aquarium will deplete the oxygen levels and could adversely impact the environment for your fish and other livestock.

Phosphates are the natural by-product of the decomposition of plants and uneaten food. The latter is particularly relevant in cases of overfeeding.

Good aquarium maintenance to remove decaying matter and uneaten food, coupled with routine water changes to control any build-up, should keep phosphate levels under control and eliminate the need for regular testing. Although, you should note that phosphates can also be present in some water supplies and thus can contribute to the build-up in the aquarium water. In these circumstances, it is worth establishing the phosphate level to take appropriate measures.

Carbon Dioxide (CO2)

In a typical freshwater aquarium, the levels of dissolved carbon dioxide are relatively low and not generally of any consequence.

In a heavily planted aquarium or aquascape, you may have an injection system incorporating carbon dioxide into the water to help feed your plants. However, high levels of carbon dioxide are deadly to your fish and livestock, so it is imperative to maintain the correct balance. In these cases, a visual carbon dioxide monitor is usually used to monitor the levels.

The level of dissolved carbon dioxide in your aquarium water is affected by several factors, predominantly carbonate hardness (KH) and pH. With carbon dioxide injection, the pH will drop due to carbonic acid (H2CO3) formed from the carbon dioxide. However, more typically, the equilibrium would be as follows.

CO2 + H2O <=> H+ + HCO3

We have discussed the two parameters KH and pH above, and monitoring their values is also helpful in this instance. Look-up tables are available to determine dissolved carbon dioxide at a given pH and KH value.

How often should I measure my aquarium water parameters?

Hopefully, it is clear from the above that achieving an appropriate balance in your aquarium's water chemistry is fundamental to creating a stable environment for your aquatic livestock.

Therefore, it makes sense that you should regularly monitor the critical water parameters, but how often should that be? Many fish keepers have differing and sometimes very opposing views on this subject.

Most aquarists will agree that testing key water parameters is critical for success when establishing a new aquarium setup.

Situations like old tank syndrome, where water chemistry parameters slowly change over time, potentially leading to a sudden catastrophic collapse in the aquarium ecosystem and death of livestock, also suggest regular testing is beneficial.

Some fish keepers argue that routine testing of the water parameters of an established aquarium is not required. They suggest regular observation of the behaviour of your fish and other livestock is all that is necessary to 'see' if anything is wrong. Indeed, whilst daily monitoring of your livestock's health is vital, changes to the water chemistry will often be invisible and go unnoticed until it is too late. Hence, testing for the above-mentioned key water parameters during your monthly aquarium maintenance sessions is considered good practice. Remember this is in addition to your daily checks, for example, fish health, removal of decomposing materials, correct operation of equipment, water temperature etc.


Understanding the natural processes and the critical water parameters of your aquarium has hopefully answered why the chemistry of your fish tank water is fundamental to your aquarium's aquatic ecosystem and the health and well-being of your fish and other livestock.

Our objective in this article was to provide an overview and identify the critical water parameters when keeping a freshwater aquarium. The additional articles hyperlinked in this text will help give a more detailed, in-depth understanding.

Measuring critical parameters such as the concentration of ammonia, nitrite, and nitrate, together with values for pH, carbonate hardness and general hardness, will help keep your aquarium and fish healthy.

If you are confused about the wide range of available test kits, especially for beginners, I would recommend the Aquarium Lab multi test-kit . If you want to learn more about the pros and cons of various test kits, including automated testing equipment, our follow-up article on freshwater aquarium testing equipment will help.

Author: Stewart Kessel CChem, MRSC

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