Aquarium Water Hardness. A Quick Guide.

Water hardness refers to mineral concentration in the water. Although it’s not the first thing to worry about, it helps to have a basic understanding of your tap water’s hardness.

While some species are native to soft water areas, many are well adapted to harder water due to breeding in captivity. Betta fish don’t mind some extra calcium or magnesium, so you don’t need to worry about this parameter as much

Water hardness in USA

Most tap water in the US is moderately hard to very hard, as it passes through calcium and mineral deposits before reaching the public system.

Tap water is treated to eliminate bacteria and harmful chemicals, but nobody touches its hardness.
You can find a map of water hardness in the USA here.

What is hard or soft water?

Water contains various chemicals and minerals in different concentrations. We’re looking at minerals’ concentration when talking about water hardness, primarily calcium and magnesium. A high concentration of these makes the water hard.

On the other hand, when calcium and magnesium levels are low, we are talking about soft water.

What about salt and water hardness?

By adding salt water gets softer as explained here. Small quantities of salt will not harm your fish and are helpful for sustaining breathing functions, but you should test the water anyway, especially if you have a freshwater aquarium.

Salt and mineral deposits.

Salt and mineral may build up on your tank walls and create long-lasting deposits if you don’t clean them regularly.

For empty tank walls, the easiest way is to disassemble them and layer them with towels soaked in vinegar for a few hours. After that, use a single-edged razor blade to scrape off the deposits. Finally, clean the bonding edges with acetone before reassembling the tank.

Signs of water hardness.

You can tell if your tap water is hard by a few clues.

  • Residue on your washing machines, dishes or laundry.
  • Spots after you cleaned your kitchen or bathroom surfaces, or after a shower.
  • Feeling your hands scrummy after washing, due to the soap reacting to the calcium in water.
  • Faster clothes’ wear due to the harsh minerals.
  • Decreased water pressure due to mineral deposits in the pipes.

Signs of soft water.

  • First, none of the above applies.
  • Drinking water has a slight sodium taste.
  • Soap creates a healthy lather when washing your hands, body or dishes.
  • You have Constant water pressure.

Water hardness and pH.

Hard water usually has a high pH (alkaline), while soft water has a low pH (acidic). You can read more on betta tank pH here.

The difference between GH and KH.

GH and KH are the two types of water hardness measurements. They show the concentration of different minerals.

GH (General Hardness) measures the number of dissolved calcium and magnesium ions in the water, in “parts per million” (ppm).

Hard water has over 200 ppm and 12 – 20 GH, while soft water has under 135 ppm and 4 – 8 GH.

You can measure it with the help of a “general hardness kit” from the pet store. You can also call your water utility, as they have to monitor these parameters.

KH (Carbonate Hardness) measures the exact levels of carbonate and bicarbonate ions. It is valuable since it shows the level to which the water is able to withstand a sudden increase in acidity.

Most species will easily adapt to a wide range of GH, but are more sensitive to KH variations, especially when it gets very low. A low KH means that the water pH may fluctuate easily, which can kill them instantly, or at least make them sick.

How to monitor water hardness.

GH and KH are both measured in similar ways, usually with the help of a liquid test kit. There are several manufacturers that sell complete KH and GH testing kits.

You run the test by putting a small quantity of tank water in a glass tube. Add one drop of testing solution at a time, until the water turns blue. After each drop added, put the cap back on and invert the tube to mix the water and solution together.

Keep adding drops until the water turns yellow. The number of drops used to turn it yellow is the dKH (degree of KH).

It’s the same for the GH test, just that the water color will change from red to green. As with the KH test, the dGH is the number of drops used to turn it green.

How does hard and soft water affect fish and aquarium life?

Different species require different water chemistry parameters. As a general thought, it’s usually safer to have harder rather than softer water, as hard water contains more necessary minerals for most species.

Most freshwater fish prefer moderately harder than softer water. A decent water hardness level is important for a healthy life, as the salts and minerals support organ function and muscle growth.

On the other hand, some species will only breed in soft water.

Please do some research about the particular needs of your favorite species before adding it to your tank. Pay attention to some more sensitive tropical species, as they will not survive in hard or moderately hard water.

Adaptation to hard water.

Most aquarium fish are grown in captivity in water from standard water facilities. However, as most tap water is rather hard than soft, they are well adapted to living in the “wrong” type of water after just a few generations.

Given that they were raised in hard water conditions, it makes sense to use the same water type for your own tank rather than try to change their living conditions.

Species that like hard water.

Some species that thrive in hard water are: shrimps, mollies, guppies, platies, swordtails, Paradise Fish, some cichlids, or brackish species like Monos, Archers and Scats.

Snails especially need hard water for shell development. Soft water will prevent it from developing.

Bettas prefer softer water, but will adapt well to living anywhere between 5 – 20 dGH (70 – 300 ppm). However, very hard water can cause them harm.

On the other hand, very few species need specifically soft water. Wild-caught Discus is one such example.

What about Plants and algae?

Plants are not so much affected by GH. Generally, most plant species will be happier in softer water, around a GH of 5 and KH of 1.

Some species that thrive in hard water are: Rotala “Sunset”, Cryptocoryne, Bacopa Caroliniana, Pogostemon Helferi, the Amazon Frogbit and some Vallisneria species.

Plants like Anubias, Java fern, and some Swords and Crypts will do well as they are quite hardy, but a lower dKH (between 12 – 18) will make them more comfortable.

There are very few plant species that are strictly soft water plants. Most plats of the Eriocaulaceae family will only live in soft water, as will all species that need constant CO2 injection.

Algae thrive in alkaline water, with a pH between 7 – 9. As hard water has a pH of around 8.5, it is in the optimum range for algae growth. Algae are not affected by changes in water hardness. They thrive on excess nitrates, phosphates, and CO2.

Water hardness change over time.

Water hardness may change over time. In a tank, plants and fish may consume the minerals in water over time, resulting in softer water. This is a natural process and it won’t affect your tank life greatly, although your fish will be grateful for some extra minerals from time to time.

How to soften tank water.

Please be very careful when softening the water, as it is best to do it before adding the fish. If you do it after adding the fish you must do it slowly, lowering the GH no more than 1 degree per day.

Peat moss, leaves (especially Indian almond leaves) or driftwood work like efficient water filters that trap minerals and lower hardness.

Some aquarists recommend adding baking soda to hard water, about 1 tsp per 5 gallons. Always check the hardness after adding it, and only add more if needed. It’s better to do it a little at a time.

A free and handy solution to soften your aquarium water is rainwater. It is available in any quantity for free. You just need some pots to collect it. Beware, though that in polluted areas, rainwater may be acidic.

To soften your tank water you can add reverse osmosis (RO) water.

Reverse osmosis is a water purification process in which water passes through a semipermeable membrane that filters out contaminants and sediments at particle level. It can be used to clean chlorine, salt, dirt, minerals and even microorganisms out of tap or well water. Through reverse osmosis you are basically left with just pure H2O.

However, be careful as reverse osmosis will strip water from any sodium. It is wise only to use a smaller quantity of reverse osmosis water to lower the water hardness rather than use exclusively reverse osmosis water for the whole tank.

What about tannins?

Liquid tannins can also help soften the water. Many substrate types and decorations will naturally leach tannins in the water, like Indian almond leaves or Malaysian wood. Tannins will give water a dark brown color, sometimes called “blackwater”.

Bettas and other Asian fish species thrive in blackwater. Tannins will also lower the water pH, making it fairly acidic. No worries, this is a natural thing in a Betta’s natural habitat.

How to harden tank water.

There are a few ways to increase water’s mineral or calcium concentrations to make it harder.

Shells, coral, oyster shells or dolomite, release minerals and carbon ions into the water. You can add these in your filter, in a bag placed in your tank, or just mix them with gravel.

Limestone in water will release calcium, increasing its concentration in the tank.

Add in more tap water: as tap water is naturally hard (or usually harder than that of your tank).

Commercial additives sold in pet stores are also a handy solution.

Substrates that affect water hardness.

You can use substrates to effectively adjust water hardness as needed. While leaves and wood help lower the water GH, crushed shells help increase hardness for species that benefit from the extra minerals.

As discussed earlier, there are a few substrates we can use to increase the water hardness, like: crushed coral or shells, as well as aragonite. Aragonite is a substrate primarily used for African cichlid tanks, as they specifically need hard water to stay healthy.

On the other hand, substrates like volcanic ash, leaves or driftwood will help keep water hardness lower, also buffering pH changes. Note that leaves or wood may leak tannins into the water, which can make it darker. While it is not vital for your betta, it is a nice way of displaying it in a more native-like environment.


Water hardness is an important parameter. While most water available in the public system is hard, it doesn’t seem to bother fish as much as we’d think. Most species are well adapted to living in hard water, even though it is not their natural habitat. Usually, harder is safer than softer unless you have very sensitive, soft water species.

About the author

Hi, I am Laura. I love fish and pets in general.
I have researched aquariums extensively. I have worked in pet stores, helping customers do the right choices. I am also the owner of two cats.

You can find the articles I wrote here

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