How To Soften Aquarium Water

by Aquarium Scoop | Last Updated: August 4, 2021

Wondering how to soften your aquarium’s water? Is it even necessary?

In this guide you’ll learn:

  • What is water hardness?
  • Why should you soften your aquarium water?
  • How do you test your water hardness?
  • How to soften your tank’s water
Guide to softening aquarium water

In aquarium keeping, you will likely come across the term “hard water” or “soft water” especially concerning the water coming from your household tap. These are terms used to describe certain properties of water that may make it unsuitable for aquarium use. Normally, you will have to employ some kind of treatment to soft or hard water to make it suitable for your tank. 

What Exactly Is Water Hardness?

Water hardness is a measure of the amount of dissolved minerals in the water, specifically calcium and magnesium. These calcium and magnesium minerals are usually found in their bicarbonate, carbonate, and sulfate forms when dissolved in water.

Washing with soap and hard water often leaves a film or slimy residue on your hands which is calcium reacting with the soap forming “soap scum.” Sometimes, a white residue is left on dishes and glasses washed with hard water. These residues can also form in pipes, your electric water kettle, and even your coffee maker.

Hard water forms when water passes through deposits of limestone, chalk, or gypsum in certain rocks at its source or spring. Soft water, on the other hand, is water with low levels of calcium and magnesium minerals.

Water hardness is usually given in degree of hardness (dh) or ppm representing the amount of dissolved minerals. Specifically, hardness can be given as general hardness (GH) or carbonate hardness (KH). GH is a measure of dissolved calcium and magnesium in the water and KH is a measure of carbonates (CO32-) and bicarbonates (HCO3) ions in the water.

Reasons Why You Should Soften Aquarium Water

Most aquarium fishes will actually thrive and adapt in most household waters as long as it is treated for chloramine and chlorine. However, there may be some specific reasons for certain aquarium keepers to soften their water. 

You intend to keep fish that requires soft water

If you want to take care of fishes that require very soft water then you will need to make sure your water is adequate for their survival. Some delicate fishes cannot adapt to hard water. These are often species kept by advanced aquarium keepers such as wild Discus and Apistogramma, some Rasboras and sensitive Gouramis and Chocolate Gouramis. Keeping soft water can be quite tedious so these sensitive fish species are best for expert hobbyists. 

You intend to breed certain fish

Water hardness can sometimes be critical to breeding fishes. Even if certain fishes like tetras and South American dwarf cichlids do well in hard water aquaria, they will not show any reproductive behavior unless they are in soft water. Even if they deposit their eggs, these may not develop due to the wrong water chemistry. If you want to get into breeding, you may have to transfer your fish into a soft water tank or adjust your tank’s water accordingly.

Tropical fishes come from areas with soft water

Most aquarium fish are naturally from soft water environments but they do thrive in hard water successfully. Unless you have a specific species that need soft water, most common aquarium fishes (apistogramma/cichlids, discus, rasboras, and tetras) are suited to moderately soft water conditions with a hardness of 5-10 dGH and a pH of 6.5. 

That being said, most tropical fish will actually do fine in household tap water provided that these are treated to remove chlorine and chloramine.

Brighter colors

Some freshwater fishes such as the Apistogramma, Cardinal Tetras, African Killifish, and Harlequin Rasboras look a lot better in soft, acidic water with a slight tinge from tannic acid (tea-like color). Hard water sometimes mutes or dulls the color of these fishes. 

Dwarf cichlids swimming in an aquarium with soft water
The color of fish like Dwarf cichlids (Apistogramma) can appear brighter in softer aquarium water.

Precautions In Keeping A Soft Water Aquarium

Soft water has low levels of dissolved minerals. Ions such as carbonates and bicarbonates associated with calcium and magnesium minerals (alkalinity reserves) that are rich in hard water provide a buffering capacity that allows the water to resist changes in pH by neutralizing acids. Hard water or water-rich in these minerals are often more pH stable than soft water. 

Both hard and soft water (or any type of aquarium water) will have sources of acidity which include: nitrification which produces nitric acid, respiration as organisms “breathe” and produce carbon dioxide, photosynthesis from plants, and tannins from driftwood and peat. The difference is how the water reacts to these sources of acidification. Soft water has limited alkalinity reserves that can provide a buffering capacity to counteract drastic pH changes. As a result, soft water tends to be more pH unstable than hard water.

This means that you will have to pay closer attention to your water quality parameters when keeping a soft water aquarium. You may also have to do more frequent water changes to keep replenishing your water’s alkalinity reserves. 

How Do I Know If My Aquarium Water Is Hard?

Use test kits

The easiest way to check for hardness would be to use an aquarium water hardness testing kit which is readily available in stores. These are usually test strips and reagent tests that you perform on a sample of your aquarium water. More accurate digital probes are also available.

Strip tests change color depending on the hardness of your water. You dip it in a small subsample of your aquarium water, wait a few minutes, then compare it with a color chart. 

Reagent tests come with a solution you add to a subsample of your aquarium water. You add a few drops (check the instructions) of a reagent to your water and wait for a color reaction. You then compare this color with a chart.

These test kits will usually give you a range of water hardness in different units.

Degree of Hardness (DH)ppm equivalentHardness description
0 – 30 – 50Very soft
3 – 651 – 100Soft
6 – 12101 – 200Slightly hard
12 – 18201 – 300Moderately hard
18 – 30301 – 450Hard
30 +450 + Very hard

Remember to throw away your subsample and do not use the tests directly into your aquarium. Take note of the expiry date of these tests as well. 

Perform a soap test

A quick visual test you can do on your water source is a soap test. Take a clear water bottle or glass and fill it with 1 ½ cups (350 ml) of water. Add 10 drops of any liquid soap then shake or stir your soap solution. If a 1 inch layer of suds or bubbles form, then you have soft water. If the solution turns cloudy or doesn’t have a lot of suds on top, then you have hard water. 

Check for a white residue 

If you have hard water at home, your pipes, drinking glasses, and other appliances (such as your water kettle or coffee maker) may develop a white residue over time as the carbonates in hard water precipitates out. You may also have cloudy water or your aquarium glass may become cloudy over time. If a whitish residue develops along your tank’s water line then your water may be hard as well. 

How To Soften Aquarium Water

Use Rainwater

One of the easiest and most sustainable water sources for aquarium use is rainwater. A lot of tutorials are available online for a DIY rainwater catch. Rainwater is usually soft. However, you also have to make sure that the air quality in your area is good so that the resulting rainwater is not too acidic. 

Make sure your gutters are also free from any substances that are toxic and may dissolve in rainwater. A good carbon filter is the best way to pre-treat rainwater for aquarium use. Monitoring its water quality is also recommended before using it in your aquarium. 

Dilute hard water using distilled water

Another method to soften water for aquarium use is dilution. If you have hard water coming from your tank, you can augment this with bottled or distilled water (check that it is distilled and not mineral water) to dilute the dissolved minerals and make it soft enough for your purpose. Make sure to test the water parameters before using in your aquarium. 

Use reverse osmosis filtered (RO) water

Using RO filtered water is a popular, efficient, and effective way to soften your aquarium water. If you are serious about aquarium keeping, a reverse osmosis water purifier is a great investment you can make. This will give you clean water consistently that is free from heavy metals, minerals, and contaminants. 

Use water softening pillows for your filter

You can install water softening pillows in your aquarium filter that helps soften your water. These can be recharged and reused but are usually best for smaller gallon aquariums.

Using a peat filter

Peat can also be used in your filter to trap the minerals that cause water hardness. This isn’t as effective as the methods mentioned above and may not do well with really hard water. Place peat in a filter bag then place it in your filter. It’s good to boil your peat first to make sure there are no contaminants. Make sure you are purchasing pure peat with no additives as well. Alternatively, you can also soak large batches of peat with an aerator for a few days to produce soft water for your aquarium.

Expert Tip 🧠

The most common causes of browning plants are pests, diseases, and nutrient deficiencies.

Tips In Keeping A Soft Water Aquarium

As mentioned, soft water aquariums will have a harder time buffering or resisting changes in pH. Because of these, soft water aquariums should be kept as inert as possible (no other substances to cause a chemical reaction). Substances rich in calcium carbonate such as coral substrates or shells should be avoided or kept to a minimum as they will raise hardness when they dissolve. Substances that release tannic acid such as peat moss and driftwood should also be avoided.

It’s quite important to keep the processes in your aquarium that promote acidification in check. Since soft water has limited alkalinity reserves, the pH can quickly lower and affect your fishes and other tank inhabitants. The best way to do this is to keep your tank inhabitants to a minimum (understocking) and use aeration to release excess carbon dioxide from your water into the air. 

Avoid drastic pH changes in your soft water aquarium by sticking to a good water change schedule (every week is best). You can also use chemical buffers available in pet stores (typically made of phosphoric acid) to help stabilize your aquarium’s pH. 

With maintaining a soft water aquarium, especially for use with sensitive fish species or breeding, you should keep a good water quality monitoring schedule and have a good record of every adjustment that you do so you can easily replicate your water parameters and avoid stressing out your pets. 

Will Softening My Aquarium Water Harm My Fish?

Not exactly. Softening aquarium water means taking out or diluting the minerals that form a buffer system that can stabilize your aquarium pH levels. These are the calcium or magnesium carbonates and bicarbonates (called alkalinity reserves) that neutralizes acids preventing pH levels from going down too quickly. Because soft water has limited alkalinity reserves, it tends to be more prone to drastic or quick changes in pH, specifically acidification. 

It is this low pH or acidic water that can eventually harm your fish. Low pH levels can affect your fishes’ reproduction, growth, and ability to recover from illnesses. 

That being said, soft water will benefit fish species that are not built and will not survive hard water. Soft water is also useful to fish breeders as certain fish species will only show reproductive behavior in soft water.


Will hard water kill fish?

No. Hard water can be harmful to fish over time but most aquarium fishes are actually adaptable to hard water. Proper monitoring of your aquarium’s water quality and frequent water changes to keep the chemical balance will help keep your fish healthy.

Will softening my aquarium water harm my fish?

No, as long as the soft water is properly maintained. What may harm the fish is the inability of poorly maintained soft water to keep a stable pH. Drastic changes in pH affects the reproduction, growth, and ability of fishes to fight off diseases. 

Does aquarium salt soften water?

No. Aquarium salts have a negligible effect on water hardness. Aquarium salts may raise the pH of water for a limited time but pH level does not always relate to water softness or hardness. 

Does boiling water remove hardness?

Yes. Boiling water will remove some hardness (called “temporary” hardness). Temporary hardness refers to the calcium and magnesium carbonates in hard water while permanent hardness refers to the calcium and magnesium sulfates in hard water.

Boiling water will remove the calcium and magnesium carbonates by allowing it to precipitate. If you notice white sediments while boiling hard water, take these out or filter them out to remove some of the hardness.

How do you make tap water safe for fish?

The best way to make tap water safe for fish would be to treat it to remove chloramine and chlorine using water conditioners. Another good way to prepare tap water for aquarium use would be to aerate the water for a few hours and stock it for at least 24 hours before using in your aquarium.

Is water from a water softener safe for fish?

Domestic water softeners may not be safe for use with fish or other aquarium inhabitants. Even water softeners from pet stores only provide a short term solution to hard water and are not always safe for fish in the long run.

Can I use distilled water to soften aquarium water?

Yes, you can use distilled water (not mineral water) to dilute your hard water and make it softer.


Softening water for aquarium use is not easy to set up or maintain and can be quite expensive. There are also certain precautions that should be taken such as keeping a better eye on your tank’s nitrogen cycle and pH levels since soft water can succumb to acidification more quickly. 

However, if you wish to keep certain fish species that only thrive in soft water or perhaps go into breeding, you should prepare to set up and keep a soft water aquarium.

Further Reading

Brief educational article on water hardness:
USGS: Hardness of Water

Collection of articles on water hardness
Water Hardness – an overview

Informative article on soft water aquariums:
The Soft Water Aquarium: Risks and Benefits
Aquarium Water Quality: Total Alkalinity and Hardness

How to measure water hardness:
4 Ways to Measure Water Hardness