Aquarium Water Hardness

gold neon tetra fishAquarium water hardness is a part of the aquarium water chemistry that is often not fully understood. In all, it is not that complicated. Fish tank water hardness is measured in degrees of hardness. Many home aquarium water test kits will give you measurements in either Degrees of Hardness (dh) or in parts per million (ppm).

When we discuss aquarium water hardness, we are simply looking at the amount of dissolved minerals in our aquarium water. There are two distinct measurements of aquarium water hardness. When you test your water you will test for General Hardness (GH) and Carbonate Hardness (KH).

General Hardness (GH)

The General Hardness (GH) of your aquarium water is a measurement of dissolved magnesium and calcium. The (GH) of aquarium water can have effects on your tropical fish so it is very important to ensure that the tropical fish you choose to keep will thrive in your water.

There are species of tropical fish that live in soft water and others that live in hard water. Like with many other aspects of tropical fish keeping, I suggest you research what type of fish you would like to keep and then test your water before purchasing any tropical fish. This will ensure that you do not put a tropical fish in danger or lose your money when fish begin to die.

Most tropical fish will survive in the average water from our tap (chlorine and chloramine removed) with no problems. Unless you test aquarium water hardness and notice that you have extremely hard aquarium water or extremely soft aquarium water you should be just fine. Please note that when you read Tropical Fish Profiles and the fish you really want to keep must have soft water and your water from the tap is very hard, your fish may become sick, or even die. Some fish will adapt just fine. I will tell you that successful breeding and the overall coloration of the fish can be linked to the proper/natural water hardness.

Carbonate Hardness (KH)

The second component of Aquarium water hardness is the Carbonate Hardness (KH). (KH) is the measurement of carbonate and bicarbonate ions in your aquarium water. In simple terms, the (KH) of aquarium water is a measurement of the buffering capacity of your aquarium water. The (KH) of aquarium water will determine how much your Aquarium pH will fluctuate. The higher your (KH) is in your aquarium water the more stable your pH will remain. When (KH) drops, so too will your pH. There is a distinct relationship between (KH) and Aquarium pH. So if you keep having problems with your pH dropping, look to see what your (KH) levels are in the tank water.

General Hardness Chart

Degrees of Hardness (DH) ppm Hardness
0-3 0-50 Very Soft
3-6 51-100 Soft
6-12 101-200 Slightly Hard
12-18 201-300 Moderately Hard
18-30 301-450 Hard
30+ 450+ Very Hard “Liquid Rock”


How to Soften Aquarium Water

There are several ways that you can soften your aquarium water if your water from the tap is too hard for your tropical fish. I would like to state that if you do need to soften your aquarium water, please do so slowly. Remember that any drastic changes can send your tropical fish into shock, cause injury, or even death.

R.O. Units (Reverse Osmosis)

The most economical and most popular way to soften your aquarium water is to use an Reverse Osmosis (R.O.) Unit. These units remove heavy metals, minerals, and contaminants from your water source. Most use a large Rubbermaid garbage can (clean of course) to store R/O water in. You can purchase water containers to store your R/O water in if you choose. You may have to experiment by mixing R/O water with plain tap water to get the exact hardness you desire. Stay consistent, write down what you do to ensure you can match it every time you change your aquarium’s water.

Water Softening Pillows

There are several different manufactures of water softening pillows that you can install in your aquarium filter. These pillows work well for smaller aquariums, but do not do that good of a job with large aquarium tanks. You simply lay them in your filter and as water passes through the pillow material, it softens the water. Many pillow softeners can be recharged and reused.

Rain Water

Many tropical fish keepers prefer to use pure rain water. If you choose to catch rain water for aquarium use, ensure that you have clean air quality. Also, please ensure that when you set up your rainwater catching system that all aspects are sterile and clean. Ensure that the gutters and roof do not contain any chemicals that may leach into your water.


Peat is often used by tropical fish keepers to help soften the water. Many still put the peat in their aquarium filter so that the water flows through the peat and back into the tank. Others prefer to get a large container or bucket and soak the peat for 1-2 weeks. Many put the peat in a pillow case and submerge it in the bucket of water. You will need to add an airstone so the water is aerated and constantly moving.
**Boil the peat first to remove any contaminants. Ensure you purchase pure peat with no additives.


Adding driftwood will help soften your aquarium water. It will not as well as those mentioned above, but it will help none the less. Be sure to use clean driftwood. You can boil smaller pieces to ensure you remove any toxins. To clean larger pieces, poor boiling water over the wood and when it cools, scrub with a brush. (Be careful boiling water is HOT!) Driftwood will release tannins in your aquarium water. This will not hurt your fish, but will stain your water and make it look brownish. You may want to soak your driftwood in a separate container for several weeks before putting it in your tank. If you choose to soak the driftwood, add a powerhead to or airstone to keep the water moving. Change the water daily. If you do not, the water will become stagnant and a slime will form on your driftwood.

How to Increase Aquarium Water Hardness

Depending on where you live, your water source may be too soft. You may need to harden your aquarium water. There are several things you can do to harden your aquarium water.

Crushed Coral

Adding crushed coral to your tank can help increase the aquarium water hardness. Many choose to fill filter media bags and add the crushed coral to their filters. Some choose to use crushed coral as their substrate instead of gravel.


Many tropical fish keepers choose to use different forms of limestone in their aquarium. Using limestone allows for calcium and other minerals to leach out into the water column. Using limestone rocks also gives your tank a cool look and provides shelter for your tropical fish. The most popular limestone used in cichlid tanks is Texas Holey Rock Limestone.

Buffer Additives

There are several buffers on the market that will help raise GH and KH while maintaining the pH of the aquarium water. Most use a combination of different salts including carbonate salts to increase aquarium water hardness. Ensure that you follow the directions on the manufacture’s label.

Aquarium Water Hardness Tips

  • Even though some tropical fish require a certain type of water, most will adapt to the average water we have in our homes. Unless you have extremely hard or soft water, or an exotic fish that has very specific requirements, you should be just fine.
  • Maintain a strict schedule of water changes to ensure high water quality and this will prevent your pH, GH, and KH from swinging back and forth so much.
  • If you decide to dose additives, be sure to follow the directions on the manufacturer’s label. Remember you must dose each time you change your aquarium water.
  • When you use driftwood, peat, limestone, or crushed coral ensure that you clean it throughly.
  • When adjusting any water parameters such as Aquarium Water Hardness, adjust slowly. You can kill your fish with extreme changes.
  • Stay Consistent in whatever you do. If things are going good, do not change things up and stay on a regular schedule.
  • Purchase a high quality Aquarium Water Test Kit. This can be one of the best investments you make in your tank!

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