Aquarium Guide to Water Quality: Water Changes, Treatments, and Chemistry

Written by Sarah Psaradelis
Updated: July 12, 2023
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When it comes to owning an aquarium, the water quality is incredibly important. The aquarium’s success is largely based on the water quality and how well you maintain it. Many novice aquarists make the mistake of assuming that the water quality will sort itself out, which is untrue. Your responsibility as an aquarist is to maintain and adjust the water quality to create a balanced ecosystem within those glass walls. With this in mind, we have created a go-to guide on aquarium water quality.

Understanding Your Aquarium Water

African Cichlid Fish Tank

The aquarium will be your fish’s home. It is important to make sure that the environment is ideal for them.

©LizWinfreyV, CC BY-SA 4.0, via Wikimedia Commons – License

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Think of an aquarium as a small and contained environment, sort of like a mini ecosystem. This is exactly what it will be for any live inhabitants inside. When you add the water and necessary equipment (filters, heaters) into the aquarium, you are starting to create an ecosystem.

This will be your fish and live plants’ home. It is essential that you ensure any live inhabitants in your aquarium have the right water quality to survive and thrive.

Why Water Quality is Important

The water quality plays a major role in the success and health of your aquarium inhabitants. Because your inhabitants will live, breath, and defecate in the aquarium, you’ll have to maintain the water. Aquarium inhabitants rely on specific water conditions to thrive in their environment. Each fish has certain water quality requirements.

If you keep aquarium inhabitants in the wrong water conditions, they will be susceptible to disease and may die. This is why it is important to understand the different water requirements of any fish or plants you choose for your aquarium.

For example, a freshwater fish wouldn’t be able to thrive in a marine environment. Freshwater inhabitants need water with little to no salinity content, whereas marine inhabitants need the opposite. Furthermore, tropical fish require a heated environment and water that is much warmer than cold-water fish do.

Cold-water fish have not adapted to the lower oxygen levels in warmer waters. This is because warmer water holds less dissolved oxygen than colder water. If the fish has adapted to a colder environment, placing them in warm water can interfere with the amount of oxygen they take in.

What’s In Your Aquarium Water?

Group of colorful discus (pompadour fish) are swimming in fish tank. Symphysodon aequifasciatus is American cichlids native to the Amazon river, South America,popular as freshwater aquarium fish.

There are many different types of aquarium fish, each with their own water quality preferences.


Whether you add tap water, reverse osmosis, well-point, or filtered water, each type has a unique chemistry that isn’t visible to the naked eye.

Different types of water can vary in their mineral traces and contaminants. They each have different water parameters and dissolved solids.

Let’s take a look below at the common types of water used in aquariums.

Tap Water

For starters, tap water safe for household use generally contains chlorines, chloramines, and heavy metals. These water components are toxic for many fish, invertebrates, and even some aquatic plants. Tap water also contains various trace elements in the water that are beneficial to certain species of fish and plants.

Due to the different elements in tap water, that can be harmful to fish, you will need to treat tap water with a dechlorinator. The dechlorinator will neutralize chlorine and chloramines. Some dechlorinators will also neutralize heavy metals, which is a bonus. The chlorine in tap water doesn’t necessarily affect fish right away, but it can have long-term effects on your fish’s health.

Reverse Osmosis (RO) Water

This is water that has undergone a purification process. Aquarists may choose to use reverse osmosis water instead of tap water because it has fewer dissolved solids and impurities. Unlike tap water, reverse osmosis water does not have heavy metals, harmful contaminants, and less chlorine. This type of water is also best for marine or reef aquariums. Many aquarists choose reverse osmosis water because it is better for many inhabitants and does not require dechlorinator treatment.

Filtered Water

Sieving or a filtration media like activated carbon creates filtered water. This is another top choice for aquarists because the filtration process removes contaminants and impurities in the water. It reduces the chlorine and chloramine levels in the water, so aquarists generally don’t need to add any water treatments.

Well Water

Well-point water can be fine for your aquarium, but you will need to test it first. Well-point water may contain harmful contaminants that can be dangerous for your aquarium inhabitants. Well water may contain agricultural or industrial runoffs and harmful volatile organic compounds (VOC) that do not make it suitable for aquariums.

Old Bathroom Sink Faucet contaminated with calcium and grime. Hard water flows from an old tap aerator.

When treated and adjusted, tap water can be safe for many fish.

©Serhii Ivashchuk/

The Basics of Aquarium Water Quality and Parameters

The aquarium water is made up of many different components that influence the water quality. These components will then influence the water parameters, which include things like pH, ammonia, nitrite, and water hardness.

There isn’t necessarily a definite good or bad water quality, since different aquarium inhabitants require different water quality than others. Water quality that could be good for one fish, could be bad for another. Usually, when we refer to water quality that is poor, we refer to water that is not ideal for the particular inhabitants for reasons such as high ammonia or unsuitable pH levels.

What Affects Aquarium Water Quality?

Once you add the aquarium inhabitants, they will start producing waste products in the water. This is usually from their poop, urea, and organic compounds from their food like proteins and amino acids. These compounds make their way into the water column and don’t have anywhere to go except to be broken down and pollute the water.

In the wild, many of the aquarium inhabitants we keep have adapted to live in large bodies of water like oceans and rivers. These natural habitats are able to dilute the waste and the beneficial bacteria can convert the waste properly so that it does not harm the inhabitants.

However, aquariums are a much smaller body of water, and the waste can build up quicker. The waste products will in return cause the water quality to become poor. This happens when the waste begins to decompose and release ammonia, which is toxic to many aquarium inhabitants even in low amounts.

Unless your aquarium has been cycled, this waste builds up over time and creates an uninhabitable environment. This is where the filter and nitrogen cycle become necessary to maintain good water quality in your aquarium.

Water Parameters:

Aquarium water parameters generally include:

  • Temperature
  • pH
  • Carbonate hardness (KH)
  • Salinity
  • General hardness (GH)
  • Phosphate
  • Ammonia
  • Nitrite
  • Nitrate
Hands holding high nitrite or ammonia test in front of freshwater aquarium. Fish tank maintenance.

There are different water testing kits that give you an indication of your aquarium’s water parameters.



Temperature is a physical water parameter that should be controlled in an aquarium. When the aquarium is left standing for a while, it is influenced by the ambient room temperature. Although some aquarium inhabitants do fine at room temperature, others do not. This is when an aquarium heater comes in handy, especially for tropical aquariums.

Not all plants and animals do fine at the same temperature, and some do better at warmer temperatures than others. You should try to keep the aquarium water at an ideal range for the inhabitants you are keeping. So, if you are keeping tropical fish, you will need to set the aquarium heater to a comfortable temperature that is ideal for their species.

PH Level

Researcher in white uniform are checking with ph strips in hydroponic farm, and pH level scale graphic, science and research concept

The pH scale ranges from 0 to 14. With 7 being neutral, below 7 being acidic, and above 7 being alkaline.


The pH level of the aquarium water is usually the easiest to adjust. It influences the acidity or alkalinity of the water which is preferable to different species of plants and animals. The pH level ranges on a scale from 0 to 14, with 7 being neutral. If the water has a pH value below 7, it is considered acidic water. Whereas if it is above 7, it is considered alkaline.

Aquarists can adjust the pH of the water using certain water treatments, although this isn’t necessary if the water you are using already has an ideal pH level for the inhabitants. Tap water usually has a pH value of anywhere between 6 to 8.5 on the scale. Whereas reverse osmosis and filtered water are usually more acidic.

In some cases, your inhabitants can adjust to a slightly different pH level than they would experience in the wild. However, you should not make any major changes to the pH once you have already added your inhabitants. This could put them into shock, and plants can be affected by this too.

Water Hardness

This refers to the calcium carbonate hardness (KH) and the general hardness (GH) of the water. The water hardness is important in aquariums because it influences the inhabitant’s growth and reproduction. The water hardness can affect plants as well and plays a role in their growth and health. Different inhabitants will have their own preferences when it comes to the water’s hardness.

The general hardness is the concentration of magnesium and calcium ions in the water, usually determined by the type of water you are using. This is when we refer to the water as soft or hard. Soft water has fewer of these ions than hard water.

Aside from a water testing kit, you can also get an idea of how hard or soft your aquarium water is by looking for a chalky white residue from any evaporation in the aquarium. Hard water is usually responsible for this residue. Some fish like the lyre-checkerboard cichlid do well in soft water, while others like live-bearing fish do better in hard water.

The carbonate hardness is measured for its buffering capacity in aquariums. This is because the carbonate hardness provides the water with carbonate and bicarbonate ions that help to neutralize acids in the water and help maintain a stable pH. Generally, as the water carbonate hardness rises, the pH does too.


Cute anemone fish playing on the coral reef.

Clownfish have adapted to living in water with a high salinity content.

©Kurit afshen/

This refers to the number of dissolved salts in the water. Freshwater aquariums will have a very low to non-existent salinity content. Whereas brackish aquariums will have a slightly higher salinity content, and marine aquariums have the highest. Different fish and plants will require a different salinity content to live in. A freshwater fish like a goldfish also won’t be able to survive in the same water conditions as a clownfish would.


The phosphates in aquariums aren’t overly important, but they can contribute to algae growth. Phosphates are natural products that form when organic waste is broken down in an aquarium. Although phosphates can be useful for plant growth, they can lead to the dreaded algae blooms which seem to take over aquariums.

Ammonia, Nitrite, and Nitrates

Ammonia, nitrites, and nitrates are very important parameters to pay close attention to. These are chemical compounds in aquarium water that are affected by waste products. Ammonia is released when the waste is decomposing, and it is very harmful to aquarium inhabitants. Nitrite is produced when nitrifying bacteria break down ammonia, and it is just as harmful.

Nitrate is the final byproduct of the nitrification process, and it is safer in levels up to 20 ppm (parts per million) for most aquatic life. The safest level of ammonia and nitrite in an aquarium is 0 ppm. This can be achieved by cycling your aquarium.

The Aquarium Nitrogen Cycle

The nitrogen cycle is the process where nitrifying bacteria are formed. The process involved adding in an ammonia source and allowing beneficial bacteria to establish themselves by converting the ammonia to nitrite and then to the less toxic form – nitrate. The nitrogen cycle is important if you want a healthy and balanced aquarium. This cycle can take anywhere from four to 12 weeks to complete before it is considered safe for aquarium inhabitants.

If an aquarium does not undergo the nitrogen cycle, the ammonia and nitrite levels in the aquarium can spike to dangerous levels. This instability will also cause any fish or plants to die, and it is the main cause of new tank syndrome. When an aquarium is cycling, the water will often turn a milky white color. This is an indication that the aquarium is experiencing a bacterial bloom, and the process should not be disturbed.

Once the cycle is complete, you will notice the ammonia and nitrite levels in your aquarium are entirely non-existent. You will find that the nitrate levels will be higher, and it is an indication that your aquarium is ready to be stocked. The aquarium needs a large colony of beneficial bacteria to create a balanced and safe environment. Otherwise, you will have a lot of stressed fish that will die fairly quickly from ammonia or nitrite poisoning.

Maintaining The Aquariums Water Quality

Let’s discuss how you can maintain your aquarium’s water quality.

Water Treatments

There is an assortment of water treatments available to make owning an aquarium easier. Common water treatments include dechlorinators, pH ups or downs, mineral replenishers, and even treatments for water hardness. These products are useful if you want to achieve a specific environment for the type of inhabitants you plan to keep in your aquarium. You get treatments for both salt and freshwater aquariums, and there are many brands to choose from.

Water Tests

To find the exact water parameters in your aquarium, you will need to use a water testing kit. The water testing kits are typically preferred over the strips because they are more accurate. You get water testing kits for various parameters like pH, ammonia, nitrite, and nitrate. They are also made for either freshwater or marine aquariums. You can find aquarium water testing kits from various pet stores and online.

Water Changes

Doing water changes in your aquarium allows you to remove older water and replaced with fresh water. This is especially important for newly set up aquariums which may have unbalanced water parameters. You may also need to do water changes on older aquariums to help dilute any pollutants. Aquarists don’t need to remove all of the water from the aquarium, but rather a partial part of it. Around 10% to 40% of water changes are usually ideal.

Doing large water changes can shock your inhabitants since the new water will have a different chemistry they haven’t adjusted to yet. You can use a gravel vacuum and bucket to do water changes in your aquarium or opt for a python for easier maintenance.

In Conclusion

Maintaining an aquarium is a fun and exciting experience. However, it does require time and patience for you to have a balanced aquarium. Once the water quality has been perfected in your aquarium, you can begin adding your fish and plants. Although you still might experience changes in the water quality of established aquariums, water treatments, and changes allow you to adjust the water quality to your liking.

The photo featured at the top of this post is © LizWinfreyV, CC BY-SA 4.0, via Wikimedia Commons – License / Original

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About the Author

Sarah is a writer at A-Z Animals primarily covering aquatic pets, rodents, arachnids, and reptiles. Sarah has over 3 years of experience in writing and researching various animal topics. She is currently working towards furthering her studies in the animal field. A resident of South Africa, Sarah enjoys writing alongside her pets and almost always has her rats perched on her shoulders.

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