Water hardness is among the key factors affecting the health of your aquarium. This comprises the amount and number of salts in water, including bicarbonates, chlorides, sulfates, and carbonates. Most fish in the aquarium don’t thrive in salty water conditions, thus the need to soften aquarium water.

Soften hard water with peat moss, reverse osmosis, rainwater, water-softening pillows, and driftwood. You can determine if water is hard by using a test kit, asking the city water supply, and looking at how the water interacts with soap. Soft water protects the fish and aquarium equipment.

How to test water hardness in an aquarium

As earlier stated, water hardness is caused by mineral salts in water. Fortunately, there are multiple ways of identifying whether the aquarium water is hard or soft.

You can determine if water is hard by using a test kit, asking the city water supply, and looking at how the water interacts with soap.

If it forms hard scum when used with soap, it’s hard. A testing strip is a helpful way of measuring water hardness in your aquarium. You dip the strip into the aquarium water, changing its color depending on its softness or hardness.

There are two measurements to determine water hardness: General Hardness and Carbonate Hardness.

1. General Hardness (GH)

General hardness entails the amount of magnesium and calcium dissolved in the aquarium water. The GH level affects the health of tropical fish. Therefore, acquiring the right GH level is essential to maintain and enhance such fish’s health.

Test the general hardness of water with a general water hardness test kit. Readings from 0-8 dGH indicate soft water, while higher values from 9-12 dGH indicate the water is hard.

2. Carbonate Hardness (KH) or Alkalinity

KH measures carbonate and bicarbonate ions in the aquarium water. This helps to determine the tendency of pH variation in your aquarium.

High KH levels in your Hardness imply more pH stability. If your aquarium’s KH levels are too high, you should lower the pH.

A carbonate hardness test kit is used to test the alkalinity of the water. Like in the general hardness, readings from 0-8 dGH indicate soft water, while higher values from 9-12 dGH indicate hard water.

Scaling is the most common way of determining water hardness within the tank. The presence of white residues at the bottom of the tank shows that aquarium water is hard.

This can make it hard to keep different fish breeds such as tiger barbs with discus as they have different water preferences.

The table below shows the available units of measuring water hardness using the two methods:

Units of MeasurementEquivalent to 1 degree
General Hardness (GH)Parts per million (ppm)
Degree of hardness (dGH) 
1 dGH= 17.84 ppm
1 dGH= 10mg of calcium oxide per liter of water
Carbonate Hardness (KH)Parts per million (ppm)
Degrees of carbonate Hardness (dKH)
1 dKH= 17.86 ppm
1 dKH= 17.86 mg of calcium carbonate per liter of water

How to soften aquarium water

There are 5 ways of softening aquarium water without putting your pet fish at risk, and they’re as follows:

1. Water Softening Pillows

Water-softening pillows are chemical filtration elements that reduce the aquarium’s general hardness. They contain ion-exchanging resins that consume magnesium, calcium, and other soluble ions. This exchange releases sodium ions by removing other minerals.

Amazingly, these pillows are reusable; you only recharge them by soaking them in brine (water and salt solution) for 2-4 hours. 

Softening pillows and other freshwater softening elements effectively lower GH in smaller tanks, accommodating less than 30 gallons. However, they can be used to soften water in larger tanks but may require to be recharged after every 48 hours.

2. Rainwater

Rainwater is the most effortless and inexpensive way of softening aquarium water. While you can test harvested rainwater to identify its KH and GH levels, it’s naturally soft.

If the rainwater is too soft for your tank’s conditions, you can harden it by adding tap water. However, you should take the following safety measures when collecting rainwater for your tank:

  • Ensure you sterilize the container you used to collect rainwater. 
  • “Food grade” containers are suitable since they won’t release chemicals. 
  • Ensure you use a rainwater harvesting method that won’t contaminate water. 

If you consider softening aquarium water using rainwater, it is essential to consider your location. For instance, industrial areas with high pollution levels and poor air quality aren’t suitable for collecting rainwater for your aquarium.

3. Driftwood

Driftwood softens water by releasing acids that depollute carbonate compounds. This element is associated with the downside of changing the color of the water to tea-colored. 

However, it is an effective softening agent since it releases tannic acid that boosts fish’s immune systems by guarding them against fungal diseases.

4. Reverse Osmosis (RO)

RO is a deionization process used in fishkeeping to cleanse water. This process involves various types of water through a semi-permeable membrane that seizes up to 99% impurities.

The membrane also stops prominent molecules such as magnesium and calcium, softening the water in the long run.

A RO system is an incredible investment if you have a large tank. However, if water quality and hardness are a constant problem, reverse osmosis will provide a dependable water-softening solution.

5. Peat Moss Filters

Peat moss moderates water hardness by tying down magnesium and calcium ions. This process, known as chelation, softens aquarium water by means of demineralization.

During this process, peat also discharges gallic acid and tannins. This interchange lowers PH and KH levels since the acids neutralize bicarbonate and carbonate ions in the water. Tannins are safe for fish, although they’ll slightly change the water’s color.

Boil the peat moss for about 2-3 minutes before using it as a filter media. This eliminates all contaminants and parasites that may be present. Also, soak it in clean water after boiling it to prevent the tank water from excessive yellowing.

Here are the steps to follow to soften aquarium water using peat moss in a clean container:

  • Boil the peat moss to disinfect it. 
  • Pour the brown water and store the peat moss in a sterilized tap water container. 
  • Allow the peat moss to settle for at least 2-3 days to soften the water. 
  • Utilize the softened water when performing routine water changes in your aquarium. 
  • Test the PH, GH, and KH levels in your aquarium to prevent major variations in water quality. 
  • Repeat the procedure if necessary. 

These methods allow you to easily and safely soften your aquarium water for healthier fish and other aquatic pets. 

Why do I need to soften aquarium water?

Softening hard water in an aquarium at a favorable level for fish species plays a vital role in your aquarium. Here are the top reasons why you need to soften aquarium water:

1. To ensure soft water fish flourish, rather than survive

Some softwater fish such as Barbs, Gouramis, and Angelfish can survive in hard water conditions. However, a fishkeeper who understands the difference between fish thriving and surviving can recognize the difference. 

Under quality water and healthy degrees where there are no signs of diseases, softwater fish can still demonstrate symptoms of distress in hard water. Thus, softening aquarium water will eradicate this stress parameter, boost their immune system, and deepen their coloration.

2. To breed soft water fish

Softwater fish can adapt and thrive in an aquarium where the water hardness is above their required range. This, however, doesn’t work in breeding tanks when attempting to promote the reproduction of these fish.

Optimizing temperatures and softening acidic water are essential to breed softwater fish. Also, ensure you reconcile your breeding pair before introducing them to the fish tank with differing water conditions.

3. To protect fish equipment from regular malfunctions

Domestic water softeners are good for protecting your home appliances from imminent regular misfunctions and limescale build-up. However, these softeners don’t alter functions in your aquarium; they instead substitute minerals carrying limescale with other minerals.

By using any of the discussed water softening methods in your aquarium, you will also be protecting other equipment in your aquarium, such as powerheads, air pumps, canister filters, or sumps.

4. To inhibit limescale deposits and hard water stains

Cleaning hard water aquarium stains can be a demanding task. Also, attempting to eliminate limescale build-ups can leave marks that can be easily identified under a crystal-clear view. 

Softening aquarium water to gradually moderate water hardness to make it functional for your fish helps resolve such aesthetic issues naturally.

5. Allow aquatic plants to thrive

Even with enough sunlight and nutrients in the water, the hardness may inhibit the growth of the plants in the aquarium. Since plants help keep the water cleaner, their stunted growth negatively affects water quality.

6. Regulate the pH of the Water

The same chemicals that cause water to be hard affect its pH. For this reason, regulating the hardness also helps you keep the pH within healthy levels for your aquarium pets.

A water conditioner for the aquarium can also help attain the desired water quality.

Soft Water Pet Fish Species

Softwater fish are generally natural to water streams that travel within low mineral soils. Wild-captured fish emerging from Southeast Asian, South American, or West African rivers are suitable examples. 

Soft Water FishGeneral Hardness (GH)
Barbs3-10 dGH
Angelfish3-8 dGH
Ram Chilclids3-6 dGH
Some Rasboras4-12 dGH
Discus3-8 dGH
Some Gouramis 5-10 dGH
Tetras2-10 dGH
Cory Catfish3-10 dGH
Apistogramma Dwarf Cichlids3-12 dGH

Hard Water Pet Fish Species

If you opt not to soften hard water in your aquarium, stocking it with hard water fish species will provide a suitable alternative. Although some fish thrive well in hard and soft water, the following fish species will thrive well in hard water:

Hard Water FishGeneral Hardness (GH)
Molly fish15-30 dGH
Platy fish10-28 dGH
Guppy fish8-12 dGH
Swordtail fish12-35 dGH
Archerfish20-30 dGH
Mono fish8-14 dGH
Paradise fish5-30 dGH

Get rid of hard water in your aquarium

Softening hard water in your aquarium will incredibly benefit your fish’s health. Further, it will enhance their breeding ability and the functional components in your aquarium.

This process involves various simple and practical ways that provide an effective aquarium with favorable conditions for your fish. Get into action to soften hard water in your aquarium now!


Duke University. What do you need to know about water chemistry and why?

North Dakota State University. Water Softening (Ion Exchange).

University of Florida. Aquarium Setup and Maintainence.

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