How to reduce water hardness in aquarium strategies to applyReduce water hardness in aquarium by using inert lime-free gravel as a substrate. They are ideal because they do not collect magnesium and calcium ions which are the top culprits of spiked GH levels in tanks. They are also safe, and they keep your tank water chemical-free. Keep reading to learn other easy techniques to reduce water hardness in your tank.

How To Reduce Water Hardness in Aquarium

Here are eleven strategies you can apply to reduce water hardness in an aquarium:

1. Reverse Osmosis Technique

This technique is best if you have to lower the water hardness after each water change. The RO system removes minerals and contaminants from the water, making it very clean. However, the system can make the water too soft, causing it to become an unfit habitat for some fish species.

Some fish prefer a certain level of hardness in their tank. Therefore, to get the right hardness content, you have to dilute the RO water with a bit of tap water. Similarly, you could use a remineralize to raise the water hardness to the ideal level.

2. Use Distilled Water

Distilled water is pure water that needs some hardness if you plan to use it in your tank. During the distillation process, the water is turned into steam. The steamed water does not contain impurities like minerals and salt. That is why you would have to remineralize the water or mix it with tap water. However, you can control the level of hardness you require in this case.

3. Use Peat Moss

When Peat moss is added to a tank it can soften aquarium water. Put some of it in a mesh bag, and then place it in the aquarium filter. Thus, when the water passes through, the peat moss traps minerals like calcium and magnesium.

Nonetheless, you have to replace the peat moss from time to time with a fresher one. Additionally, it would simultaneously lower ph levels as it contains some tannin. Nonetheless, the peat moss technique is not for extreme cases as it is not so efficient at lowering water hardness.

4. Perform Frequent Water Changes

Water changes are the fastest way for reducing water hardness in aquarium. They are very effective and would also eliminate leftover food, fish waste, and other contaminants present in your tank. This technique works by diluting the debris that causes water hardness. It replaces the tank water with a softer one making it appropriate for your fish.

For example, to make the discus fish species comfortable, siphon 25 percent of your tank water with another that has a GH below 4. Then, check to see that the GH levels have reduced. If it is still too high, continue with the water changes technique for a few days until the GH levels are ideal for the fish. However, the water changes should not be too fast that it suddenly alters the water quality in your tank.

5. Change Your Tank Water Source.

The type of water used in your tank significantly affects its GH levels. This technique is quite advanced, but you can alter its hardness if you know how different water interacts with tank chemistry. Still, if you can get soft water or water sources with lower GH, use them for your aquarium.

An inexpensive water source to use is rainwater, but you need to ensure the area has clean air. Also, the storage container should be sterile and food grade to prevent chemicals from seeping into the water. Please note that rainwater is soft, so you need to add tap water or aquarium salt to raise the GH level to your fish specifications.

6. Use Water Softening Pillows in Your Tank Filter.

You can attach softening pillows to your tank even when using the Hang-on-back filter. There are beneficial in lowering water hardness because the pillows contain resin beads that bind minerals. Therefore, it limits the ability of the minerals to bond with other ions, thereby reducing the water hardness. Note that the softening pillows are similar to water polishing filters.

They are safe because they gradually remove the minerals from the tank. You can attach them to your filter with zip ties or stuff them into the water inlet. Alternatively, place some softening pillow in the cartridge right before inserting your tank filter.

7. Add Water-Softening Crystals to Your Tank.

Softening crystals is another way to lower water hardness. They are cheap and safe because they are made explicitly for tank use. Similarly, these salt mixes are a fast fix to softening water. The sodium ions in the crystals displace the calcium and magnesium ions in the tank water, causing an ion balance shift.

Hence the hardness minerals lose affinity to bind easily with other ions in the tank water. However, you must use the crystals in the proper measurements. One liter of water softens crystals to 20-gallons of water for freshwater, while marine tanks require half that amount. Nonetheless, read the instructions as each product dosing could be slightly different.

8. Reduce the pH of Your Water Source

You can reduce the pH of your water source (tap or well) with acid buffers like gallic acid. It neutralizes carbonate and bicarbonate, reducing the water’s ability to bind to hard minerals. Acidic water holds less calcium and magnesium, causing the water to be softer.

Alternatively, you can add natural pH-reducing agents like Indian almond leaves. They contain tannic acid, which gradually removes minerals. But, they tend to discolor tank water which may not look nice. Nevertheless, if it keeps the fish happy and healthy, then it may not be such a bad trade-off.

9. Add Domestic Water Softeners

You can soften fish tank water using domestic softeners because they are safe. Although they are devices that treat human drinking water, they can do the same for aquariums. The machine replaces carbonate ions with sodium ions and reduces the GH and KH levels. However, you would need a large tank for an appropriate water volume.

Also, the extracted minerals would not be re-added to the tank, you would have to remineralize the water. Nonetheless, pour water into the container that goes into the machine, and then let the system run overnight. Set the flow to slow drip, so it gradually goes into the tank and doesn’t shock the fish.

10. Use Inert Lime-free Gravel.

Lime-free gravel is the best way to lower hardness in fish tank. They do not accumulate magnesium and calcium ions. So, to get lower GH, stay away from lime-based gravel like dolomite, limestone, or crushed coral and pea gravel.

Furthermore, lime-free gravels are chemical-free, which is ideal for fish. Some examples of inert substances include clay pellets, river rocks, and crushed granite. But having an inert substrate is not enough, you must maintain them, or they can cause high GH levels. If the inert substrate continuously traps debris like fish waste and leftover food, it would affect the water quality.

11. Remove Existing Tap Water Stains

If you allow tap water to sit in an open container for weeks, the tank will begin to have stains on its walls. These stains are calcium build-up which can cause increased water hardness levels. Overall, removing them would cause the GH levels to drop.

To Remove Water Stains From a Container, you would need Calcium-Lime-Rust remover and three percent hydrogen peroxide. Mix equal parts of water, CLR, and hydrogen peroxide in a container. Put the mixture on the build-up every five minutes for 30 minutes, which would cause the hard stains to break down.

How to Detect That Water Hardness Is Too High?

An increased water hardness level causes some problems in a tank. Here are some of them:

1. Fish Death

Fish can die if the water hardness levels are too high to handle. For instance, if a new fish is placed in a tank with higher GH levels than its previous tank in the pet store, it is likely to go into osmotic shock. You may not detect it until a few days later so, osmotic shock is likely to can cause death.

However, if they are acclimated to hard water, they would survive. To acclimate the fish, gradually add water from the tank into the fish bag for some minutes. It helps the fish adapt to the hardness beforehand, so it doesn’t get sick from the harsh conditions.

2. Impaired Disease Resistance

High water hardness levels affect fish immunity, causing them to be prone to illness. For instance, the fish would begin to exhibit sickly signs and be lethargic. This is due to the unstable KH and GH levels in its tank.

3. Increased Aggression In fish

High GH levels affect fish hormones causing them to be more aggressive towards other tank mates. The fluctuation in the tank chemistry gives the fish little or no time to adapt to changes. Therefore, they get frustrated and pass the aggression to other fish.

Furthermore, they thrive in tanks that closely mimic their natural habitat. Hence, out-of-range GH levels would cause significant fish discomfort or kill it.

4. Breeding Conditions

Fish would not breed in extremely hard water. They reproduce mostly during the rainy season when extra water has diluted their habitat. Generally, the alteration in the water chemistry triggers breeding.


1. How To Test the GH levels in Your Tank?

You can not tell the water hardness level by looking at it. You need to use an aquarium test kit specifically for KH and GH, and they are easy to use.

2. What Is the Ideal Water Hardness Range for an Aquarium?

How to reduce water hardness in aquariumThe idea of water hardness range depends on the fish species you are keeping. For example, a species that thrives in soft water should have a GH reading between 0-59 parts per million or 1-3 GH. While medium-hard water should be 60-120 ppm, any reading above 120ppm or 20+ dgh is hard water. Nonetheless, you would want to keep the GH level between 4 and 12.

3. How Often Should I Test the Water Hardness in an Aquarium

Ideally, you should test the water hardness after a water change, but every two weeks is fair. So, you do not need to test frequently. But, if you can see signs of fish distress, you should check the GH level.


The general hardness of a fish tank should not be taken with levity because your fish can die if exposed to a harsh water environment for too long. Here are some pieces of information to remember:

  • You can reduce water hardness by lowering your fish tank pH
  • Performing water changes is crucial in keeping low GH
  • Reverse osmosis is the best method of reducing water hardness

Nonetheless, be sure to monitor all water parameters so that your fish thrives. Try out any of the techniques above and lower the GH of your tank like a pro.

5/5 - (19 votes)


Please enter your comment!
Please enter your name here