Everything You Need to Know about Aquarium Water

An aquarium is not simply a glass tank with some water and fish in it, but a miniaturized eco-system that needs multiple components in order to thrive. Any successful build will therefore typically need the following mandatory components:

In this article we will take a closer look at the aquarium water, as this is one of the most crucial components to get right from the start of any aquarium build.

The Importance of Clean Water

Using clean and healthy water in your aquarium is essential for the health and well-being of all of your aquatic animals and plants. Using water that is contaminated with substances such as chlorine, lead, ammonia or nitrite can lead to diseases and even death of your animals.

The good news is: in this article we will teach you everything you need to know about ensuring you use only the best water in your tank.

Clean water is also important for aesthetic reasons. Dirty water is detrimental to the overall appearance of your tank and can lead to unsightly algae growth or other problems. Regular water changes and proper filtration can help prevent these issues and keep your aquarium looking beautiful.

Lastly, clean water is essential for the longevity of your aquarium. Dirty water can have a negative impact on your tank equipment, causing premature failure that will requiring costly repairs and replacements. You can prevent these issues by doing regular maintenance and keeping your water as clean as possible.

What kind of water for fish tanks

Fish and other aquatic animals require clean water. The water needs to be free from any harmful chemicals and heavy metals.

We always recommend to have a water test kit at home, in order to check all water parameters before adding water to the aquarium.

Your local fish store also needs water for its retail aquariums and knows best what water to use in the area. So it's always a good idea to ask there if you're unsure.

Using Tap Water for Fish Tanks

Before using tap water for your fish tank, it is important to know that the quality / parameters of tap water varies depending on where you live.

In some countries around the world, tap water can contain chemicals like chlorine and chloramine, or harmful metals such as copper or lead. All of these are toxic for fish.

If the tap water in your area is contaminated with any of the above mentioned substances, it is important to use an aquarium water conditioner that specifically removes chlorine, chloramine and any toxic metals before adding fish to your tank or adding water to a running aquarium.

Most aquarium water conditioners will remove all harmful substances. Make sure to read the instructions on the bottle first to make sure you are using the right one for your needs.

In case you're not sure if the water from your tap is fish-safe we recommend to test it via a water test kit.

Using Spring Water for Fish Tanks

Spring water is usually a viable water source for your fish tank but the quality of spring water can also vary between locations.

The quality of spring water depends on where it has been sourced and how it has been collected and processed. Spring water may contain high levels of minerals or other impurities that can be harmful to you fish.

In case you're unsure about the spring water quality or like to know if it is fish safe, you could research the brand online for more information or reach out to them directly via their website.

Using Distilled Water for Fish Tanks

Distilled water can be used for a fish tank, but it's generally not recommended as it is uncertain if all chemicals and impurities have been removed during the boiling process. It may still contain chlorine, chloramine and minerals. In this case you could just use tap water with a water conditioner removing all toxic chemicals.

If you're concerned about the quality of your tap water and are looking for an alternative source of water for your fish tank, we recommend using reverse osmosis (RO) systems to purify your tap water.

Reverse osmosis water for fish tanks

Reverse osmosis (RO) is a water purifying process suitable for any fish tank. A RO system removes any impurities such as chlorine, chloramines, heavy metals, nitrates and minerals from tap water by filtering it through various membranes. The result is an almost completely pure and PH-neutral water, making it a great starting point for fine tuning the water chemistry to the specific needs of your fish.

However, it's important to note that RO water lacks the essential minerals and trace elements that fish need to thrive. Therefore, it's recommended to use RO water only in combination with a mineral supplement to ensure that your fish are getting all the necessary nutrients.

If you're considering to use RO water for your fish tank, it's important to invest in a high-quality RO system and to test the water regularly to ensure that it's within the proper range for your fish.

It's also recommended to consult with a veterinarian or a specialist at a local aquarium store for advice on the best type of water to use for your specific fish species.

Aquarium Water Parameters

Aquarium water parameters are the different chemical and physical characteristics of the water in an aquarium that affect the health and well-being of the fish and other aquatic creatures living in it. The following are some of the most important water parameters to monitor and maintain in an aquarium.

Aquarium Water Temperature

The ideal water temperature for most freshwater aquarium fish is between 72-78°F (22-26°C) and for most tropical fish it is between 75-82°F (24-28°C).

Please always make sure to check the requirements of the species you intend to put in your tank and only put different species with similar needs together in a tank. You can check our animals page to learn more about the requirements of different species.

It's also very important to maintain a consistent temperature in your aquarium, as sudden temperature changes can stress or even kill your animals. If you cannot ensure a stable room / water temperature you might want to use a quality aquarium heater to help maintain a consistent temperature.

You should monitor the water temperature regularly, just as with all other water parameters mentioned below.

Aquarium water PH

The pH level of aquarium water is a crucial factor that must be monitored because it can significantly affect the well being of aquatic plants and animals. A pH of 7 is considered neutral and indicates that the water is neither acidic nor alkaline. A pH value of 7 or above indicates alkalinity, whereas a pH value of 7 or below suggests acidity.

For the majority of freshwater fish, the pH of aquarium water should be between 6 and 7.5. However, some fish and plant species could need a more precise pH range. For instance, while Amazonian fish like tetras and angelfish thrive in a pH range of 6.0 to 7.0, African cichlids prefer a pH range of 7.8 to 8.5. Only keep creatures in the same tank that have similar demands!

It is crucial to keep in mind that abrupt pH changes can be unpleasant for your aquatic animals, therefore any pH modifications should be made gradually.

Aquarium water hardness

There are two main measures of aquarium water hardness: general hardness (GH) and carbonate hardness (KH).

GH measures the total concentration of all dissolved minerals, mainly calcium and magnesium.

KH specifically measures the concentration of carbonate and bicarbonate ions, which play a role in pH stability.

The recommended hardness range for most freshwater fish is 4–12 dGH.

GH	Water Hardness
0 - 4 dGH	very soft
4 - 8 dGH	soft
8 - 12 dGH	moderately hard
12 - 18 dGH	hard
18 - 30 dGH	very hard

Soft water has low mineral concentration and hard water a high mineral concentration. Hard water can cause mineral buildup in the aquarium, which can lead to cloudy water, clogged filters, and other problems.

The hardness of aquarium water can affect the health and well-being of fish and other animals living in the aquarium. Some species (e.g. snails, shrimp and mollies ) and plants are sensitive to water hardness and require specific levels to thrive.

To maintain proper water hardness in an aquarium, it is important to test the water regularly and adjust the hardness if needed using products designed to increase or decrease hardness levels. It is also important to research the specific needs of the fish and plants in the aquarium and adjust the hardness levels accordingly.

Pro Tip: Water hardness can also be increased by adding crushed (clean and dry) eggshells. Water hardness can be decreased by partially (no more the 25%) replacing the aquarium water with RO water.

Aquarium Water Ammonia

Ammonia is produced by fish waste, uneaten food, and decaying plant matter. The ammonia level should always be around 0 ppm as it is a toxic substance for fish and other aquatic animals. For plants ammonia is not toxic. They even prefer / require ammonia in order to thrive.

Ammonia is the first substance in the Nitrogen Cycle and needs to be monitored closely when setting up a new tank or after thoroughly cleaning an old tank with fish.

Pro Tip: When you see the ammonia levels spike and you have aquatic animals, you should instantly make a big water change (50 % - 75 %).

Aquarium Water Nitrite

Nitrite is produced by beneficial bacteria transforming ammonia into nitrite. The nitrite level should always be around 0 ppm as it is a toxic substance for fish and other aquatic animals. For plants nitrite is not toxic. They even prefer / require nitrite in order to thrive.

Nitrite is the second substance in the Nitrogen Cycle and needs to be monitored closely when setting up a new tank or after thoroughly cleaning an old tank with fish.

Pro Tip: When you see the nitrite levels spike and you have aquatic animals, you should instantly make a big water change (50 % - 75 %).

Aquarium Water Nitrate

Nitrate is produced by beneficial bacteria transforming nitrite into nitrate. The nitrate level should range between 0 - 100 ppm (the lower the better) otherwise the amount will become toxic for fish and other aquatic animals. For plants nitrate is not toxic.

Nitrate is the third and last substance in the Nitrogen Cycle.

Pro Tip: When you see the nitrite levels spike and you have aquatic animals, you should not change more than 50 % of the water, as the aquatic animals need to get used to the lower nitrate level.

Aquarium Water Dissolved oxygen

Dissolved oxygen levels are very important if you have fish and other aquatic animals in your tank. Every living animal requires oxygen to survive. The ideal dissolved oxygen level in aquarium water should be between 5-7 ppm.

If you have a planted tank, the plants in your aquarium will provide oxygen. Another possible oxygen source is a filter or power head which ensures sufficient movement (small waves) at the upper water level promoting gas exchange. This is called surface agitation. Agitation increases the amount of dissolved oxygen in your water and decreases the amount of carbon dioxide.

Another option is a so called air stone or bubbler. This device is an air pump moving oxygen trough a hose into a diffuser (there are many different variation of diffuser) creating a lot of bubbles. These bubble will partially dissolve into the water and also promote surface agitation.

In case your aquarium is overstocked or you have big fish (e.g. goldfish) you most likely need an air stone to provide additional oxygen.

Pro Tip: If you see your fish swimming at the surface grasping for air (mainly in the morning) you most likely have an issue with the dissolved oxygen level.

Why is my aquarium water too hard?

Aquarium water can become hard due to either the type of rocks or substrate in the aquarium or the source of the water being used

One of the most common causes of hard water in aquariums is the presence of dissolved minerals such as calcium and magnesium, which can come from tap water or other water sources. These minerals can build up over time, leading to an increase in water hardness.

Another common cause of hard water also comes from the use of certain substrates and rocks(hardscape). In case there is crushed coral or limestone in either the rock or substrate, they will release their minerals into your aquarium water. In case you are using a CO² kit the amount of leaking minerals is higher.

How to soften aquarium water?

There are several options to reduce the hardness level of you aquarium water. First make sure that your tank doesn't contain any water hardening substance like limestone or crushed coral. In this case regular water changes are a must to control a stable water hardness level. Otherwise the below options should help:

Water change: Use reverse osmosis (RO) water for partial water change (max. 50 %). Reverse osmosis is a process that removes minerals and other impurities from water. By partially replacing the hard aquarium water witch RO water, you dilute your hard aquarium water which results in a immediate decrease of the overall hardness level.
Peat moss: Peat moss naturally softens the water and lower the pH by releasing tannins. It is best to put peat moss in a mesh bag and place it in your aquarium filter. The results will not show immediately, but over time.
Driftwood: Adding driftwood to your aquarium can also help soften the water. Driftwood will release tannins into the water, which lower the pH and softens the water. The results will not show immediately, but over time.
Commercial water softeners: Commercial water softeners can be add to your aquarium water to immediate decrease the overall hardness level. Make sure to follow the instructions on the product carefully.

Before using any of these options, it is important to test your aquarium water and determine the hardness level. For immediate actions, always consider a water change.

How to prepare water for fish tank?

The following steps are a guideline helping you to prepare safe and healthy water for your fish tank.

Dechlorination / Neutralizing Metals : Depending on where you live Tap water may contain chlorine, chloramines, copper and lead, which are toxic for fish and other aquatic animals. You can remove these chemicals and metals by using a water conditioner that is specialized in neutralizing them. Follow the instructions on the conditioner bottle for the correct dosage.
Testing aquarium water: Before adding any fish or other aquatic animals to the tank you should test the aquarium water for pH, ammonia, nitrites, nitrates and hardness. These levels can affect the health of your fish and should be adjust to the aquatic animals requirements if necessary.
Adjust the temperature: The temperature of the water should be appropriate for the type of fish you have. Use a thermometer to measure the temperature and adjust it using a heater if necessary. We do not recommend to add any fish or other aquatic animal the the tank before aligning the water temperature of the new fish with your current aquarium water temperature. Pro-Tip: You can lay the bag in which the new fish arrived in you aquarium to rise/ lower the temperature
Slowly add the water: By slowly adding the water to the tank you ensure not to disturb or stir up the substrate or decorations. Pro-Tip: Cover the bottom or hardscape with paper towel or plastic bag. By doing so the water force hitting the bottom will explode on the towel / bag instead on your substrate.

By following these steps, you can prepare water for your fish tank that is safe and healthy for your fish.

Why is my aquarium water green?

Green fish tank water is most likely caused by an overgrowth of algae or algae bloom. Algae needs two thing to grow light and nutrients. So you need to figure out what causes the algae to grow in your tank. There are several reasons listed below, please note that each aquarium is unique and can differ for one or many reasons.

Excessive lighting: Algae growth is stimulated by to much light or direct sun light
Overfeeding: Overfeeding your fish leads to excess food decaying in your aquarium. The decaying food leads to excess nutrients in the water.
Poor filtration: In case your aquarium filter is not functioning properly or if you do not have adequate filtration, it can result in an excess of nutrients in the water.
Lack of water changes: Not performing regular water changes leads to a build of excess nutrients in the water.
Wrong or No plants: Not having any live or fast growing plants can lead to an excess of nutrients in the water
Wrong substrate layers: If you don't have the correct substrate layers, excess nutrients can leak into the water.

You can resolve the issue, by trying the following steps:

Reduce the amount / duration of light your aquarium receives.
Make sure to keep your aquarium out of direct sunlight. In case this is not possible, check how many hours of sunlight your aquarium receives and consider the hours in your overall lighting concept. Maybe you don't need any lighting at all especially when your aquarium receives 8-12 hours direct sun light.
Adjust your feeding habits to avoid overfeeding.
Make sure your aquarium filter is functioning properly and consider adding additional filtration if necessary. A good filtration system must at least be able to filter twice the amount of aquarium water per hours (10 Gallon aquarium needs at least 20 gallon per hour filtration). In this case more is better.
Perform regular water changes to reduce the buildup of nutrients in the water.
Consider adding fast growing live plants to your aquarium, which help to absorb excess nutrients and therefor compete directly with algae growth.
In case you are using plant growth substrate or aqua soil, consider to cap it of by adding a 1 inch layer of sand or gravel. Growth substrate and any kind of aqua soil are known for being nutrient rich and leaking excess nutrient into the water.
As a quick fix: Active carbon will help to filter out all nutrients. (Should not be used permanently) Seachem - MatrixCarbon

Why is my aquarium water brown?

If your aquarium water turns brown it doesn't necessarily mean that there's something wrong with it. Some people even aim for this aesthetic when building so called blackwater aquariums. But since there can be many reasons for brown water, it is important to distinguish between the harmless and harmful ones.

Here we have compiled some of the most common reasons for brown aquarium water:

Presence of tannins: If you use specific types of wood in your aquascape, such as driftwood or mangrove roots, the water can turn brown due to the gradual release of so-called tannins. Tannins are natural compounds found in wood and can cause the water to take on a brownish hue.
Algae bloom: Brown water can also be caused by an algae bloom, which means an overgrowth of algae that can color the water brown or greenish-brown. In this case it's best to check for any visible algae growth on plants, hardscape or substrate and find out what caused the overgrowth. If you remove the reasons for the algae growth you will notice your water staying clear after several water changes.
Overfeeding: Overfeeding your fish can cause uneaten food to accumulate in the aquarium, leading to the growth of bacteria and algae. This can cause the water to turn brown.
Poor filtration: Inadequate filtration or irregular maintenance can lead to a build-up of organic waste in the aquarium, causing the water to turn brown.
Medication or chemicals: Sometimes, the use of certain medications or water conditioning products can also cause the water to turn brown. This mainly depends on the ingredients of those products and you will easily notice if they are the reason for the color change by checking your water color before and after using them.

To determine the root cause of the brown water in your aquarium consider using a water testing kit to test the water for pH, ammonia, nitrite, and nitrate levels. If the levels are within normal ranges, then the cause of the brown water is likely due to tannins or an algae bloom.

You can address these issues by adding activated carbon to your filter or reducing the amount of light your aquarium receives.

If the brown water is caused by poor maintenance, overfeeding, or medication, then addressing those underlying issues will be necessary to clear the water.

Why is my aquarium water cloudy or murky?

There are many possible reasons for your aquarium water turning cloudy:

Overfeeding: If you give your fish too much food, uneaten remnants can float around the tank or sink to the bottom and decompose, causing cloudy water. Watch your fish closely when feeding them and consider feeding less if you notice that there are a lot of leftovers regularly. You can also split their daily food ratio into more feedings per day in order to make sure they catch all the food before it sinks to the ground.
Overstocking: Always make sure to keep only as many fish as the ecosystem in your tank can handle. Putting too many animals in a single tank causes water quality problems due to excess waste (poop), which in turn cause the water to turn cloudy.
Poor filtration: Inadequate filtration can result in a buildup of organic waste in the tank, leading to cloudy water. Make sure your aquarium filter is functioning properly and consider adding additional filtration if necessary. A good filtration system must at least be able to filter twice the amount of aquarium water per hours (a 10 Gallon aquarium needs at least a 20 gallon per hour filtration). In this case more is better. Consider adding a (more powerful) filter to your tank, clean the filter or exchange the used filter medium.
Lack of water changes: Not changing the water in your aquarium regularly can cause a buildup of waste and other substances that can contribute to cloudy water. Consider changing the water more frequently (every two weeks).
New tank syndrome: When setting up a new aquarium the tank's ecosystem always needs some time to establish itself. This process is called cycling and entails the buildup of bacteria and other microorganisms, which can make the water turn cloudy. This is usually only temporary and will sort itself out with time and can be sped up with regular water changes.
Algae bloom: If you have excessive amounts of light or nutrients (e.g. due to animal waste or insufficient filtration) in the water, it can encourage the growth of algae and make the water green, brown or cloudy. In this case you need to determine the specific causes for algae growth and remove them to clear up the water.
UV sterilizer: A UV sterilizer can help to kill off any bacteria (good and bad) or parasites in the water, which can contribute to cloudy water.

To determine the cause of cloudy water in your aquarium, you should test the water quality and consider any recent changes you've made to the tank's setup or maintenance routine. Addressing any underlying issues should help to clear up the water over time.

Regular water changes and proper filtration can go a long way in preventing cloudy water and keeping your aquarium inhabitants happy and healthy.

Aquarium water cloudy after water change

It is a common issue to have cloudy water after a water change. This is due to:

Stirred up debris: When you perform a water change, you may be stirring up debris (mostly when adding new water) that has settled on the bottom of the aquarium. This debris can cause the water to become cloudy.
Dissolved minerals: The water you use for the water change may has high levels of dissolved minerals, making your aquarium water cloudy.
Bacteria bloom: The sudden change in water chemistry can cause an increase in bacterial growth, leading to a cloudy appearance.

To address cloudy water after a water change, you can try the following:

Sit it out: In many cases, the cloudiness will clear up on its own within 1-2 days as the water chemistry stabilizes.
Increase filtration: By adding additional filtration in your aquarium you increase the overall filtration resulting in less debris and excess waste from the water. This also helps to clear up cloudiness. Pro-Tip: Most aquascapers use a surface skimmer after performing a water change
Check water parameters: Test your aquarium water for ammonia, nitrite, nitrate, and pH levels. If any of these levels are out of range, adjust them accordingly.
Use a water clarifier: You can use a water clarifier product to help clear up cloudy water quickly. However, be sure to follow the instructions carefully and only use the product as directed.

Why is my aquarium water yellow?

There are several reasons why aquarium water might appear yellow. Some of the most common reasons include:

Tannins: If you use specific types of wood in your aquascape, such as driftwood or mangrove roots, the water can turn yellow / brown due to the gradual release of so-called tannins. Tannins are natural compounds found in wood, alder cones and rotting leaves and can cause the water to take on a yellow brownish tint. Tannins are beneficial to fish and other aquatic animals, but they can make your water look unattractive (black water).
Algae bloom: If you have excessive amounts of light or nutrients (e.g. due to animal waste or insufficient filtration) in the water, it can encourage the growth of algae and make the water yellow. In this case you need to determine the specific causes for algae growth and remove them to clear up the water.
Overfeeding: If you give your fish too much food, uneaten remnants can float around the tank or sink to the bottom and decompose, causing cloudy water. Watch your fish closely when feeding them and consider feeding less if you notice that there are a lot of leftovers regularly.
Poor filtration: Poor filtration will result in a buildup of organic waste in the tank, leading to yellow water. Make sure your aquarium filter is functioning properly and consider adding additional filtration if necessary. A good filtration system must be able to filter the amount of water in your aquarium at least twice per hour (a 10 Gallon aquarium needs at least a 20 gallon per hour filtration). In this case more is better. Consider adding a (more powerful) filter to your tank, cleaning the filter or exchanging the used filter medium if you're still having trouble with filtration.

To determine the cause of yellow water in your aquarium, you should test the water quality and consider any recent changes you've made to the tank's setup or maintenance routine. Addressing any underlying issues should help to clear up the water over time.

Regular water changes and proper filtration can go a long way in preventing cloudy water and keeping your aquarium inhabitants happy and healthy.

Conclusion

Maintaining clean and healthy water in your aquarium is crucial for the health and well-being of your fish, other aquatic creatures and plants. By understanding the water cycle, implementing proper filtration systems and regular water changes, you can ensure that your aquarium is a beautiful and healthy home for your aquatic pets. Remember to monitor water quality regularly, and if you notice any issues, take action promptly to prevent any harm to your fish. With proper care and attention, your aquarium can bring joy and wonder into your home for years to come.