How to lower GH in aquarium water

GH or General Hardiness is probably one of the most important aspects of your aquarium, but nobody talks about it much! It is one measure of magnesium and calcium hardness, the other being carbonate hardness.
GH is important because it can affect KH (carbonate hardness) and also affects pH (the lower the GH, the more acidic the water). Depending on the type of fish you keep in your freshwater aquarium, you will need to keep gH levels within certain parameters.

Now, let’s talk about how to safely lower gH in an aquarium. Keep reading!

What is GH (general hardness)?

General Hardness (GH) is a measure of the total concentration of calcium and magnesium compounds in your aquarium. Since both calcium and magnesium are essential for fish to live, GH levels should always be kept within safe levels: if it’s too high you can risk sick fish and if it’s too low they can eventually die.

It is known that most common species of freshwater aquarium fish perform best when the gH is between 6 and 12 dGH (degrees GH). However, there are some that require very soft water at 3-5 dGH, while others can tolerate very hard water up to 20 dGH and higher. For example, South American cichlids prefer very soft water, while some African cichlids thrive in very harsh environments.

It is highly recommended to research the type of fish you plan to keep before starting your aquarium. Doing so will save you from buying incompatible fish and you’ll also have to go through the hassle of changing the water chemistry.

What happens when GH levels are too high?

Now that we have a better idea of ​​what GH in water means, it’s time we take a look at what happens when it’s too high. Once the GH level of your aquarium water is higher than 12dGH, this can lead to several problems, such as:

  1. Lower survival rates – Some species of fish such as discus and many South American cichlids require specific levels of gH to thrive. A sudden change in water chemistry can make them feel really bad, affecting their appetite and causing stress.
  2. Impaired resistance to disease – Higher levels of GH affect the immune system of fish, making them more susceptible to disease. You will notice that your fish become lethargic for no apparent reason and generally act sickly. This is due to the rapid fluctuations of KH and gH.
  3. Increased Aggression – This is mainly due to the fact that high GH levels affect fish hormones, often leading them to become more aggressive towards other fish. Again, this is due to the rapid fluctuations in water chemistry – the fish cannot adapt properly and express their frustrations to each other!

Fish require and prefer water conditions that mimic their natural habitats. When the water hardness is out of range (either too high or too low), fish will show signs of discomfort that can eventually lead to death.

Ways to reduce GH in an aquarium

If you have fish with a preference for softer water, you may already know that keeping GH low is crucial to the well-being of your fish. In an ideal scenario, soft water would be readily available. Unfortunately, we don’t live in an ideal world and soft water can be harder to find than you think.

Because of this, fish keepers often end up having to manually lower the GH in their tank. Here are some ways you can do it:

Do frequent water changes

This is one of the easiest and safest ways to quickly reduce GH levels in your fish tank. Not only will it do wonders for your tank’s water chemistry, but it will also help remove excess fish waste, food scraps, and other contaminants. Water changes reduce the GH of the water by diluting fish waste, which is made up primarily of nitrogenous components (ammonia and nitrate).

You can lower your GH by replacing some of your tank water with softer water that is appropriate for your fish. For example, if you have discus, replace at least 25% of the water with water that has a gH less than 4. Then use an aquarium test kit to make sure your GH levels are now low enough for your fish.

If it’s still too high, you may need to change the water for a few days to bring the GH down to proper levels. Be careful not to make these changes too quickly! Although the ultimate goal is to reduce GH, doing so quickly can cause instability in water quality. This, in turn, will do more harm than good to your fish.

Change the type of aquarium water used

You can also affect gH levels in the water by using different types of water. This is probably one of the more advanced methods, but it works as long as you are careful about how the different waters interact with your aquarium chemistry. If you know specifically where to find soft/low GH water sources that are safe for your fish, go ahead and use them for water changes.

Other alternatives include using reverse osmosis (RO) or distilled water from your local pet store. This type of water is sometimes referred to as «chemically pure» water. You can also use DI water that you get from your local hardware store. The deionization process removes the ionized minerals that are responsible for the high levels of GH in the water.

Some hobbyists may wonder if drinking water is appropriate for this purpose. In general, the answer is no. For example, alkaline water is known for its health benefits when consumed by humans, but it can drastically increase the hardness of the water, something you should avoid. The hardness of tap water is also too high for most aquarium fish.

Put water softening pillows in your aquarium filter

Whether you have a cartridge filter or a hanging filter, you can attach packs of water softening pads to it. This is one of the most effective methods to rapidly lower GH levels in the tank. These pillow filters contain resin beads that are responsible for binding minerals, such as calcium and magnesium. This limits its ability to bind with other ions in your tank water, thus lowering GH.

Water softening pillows are basically the same as «water polishing» filters. By having a constant flow of water running through them, these pillows can effectively remove minerals and ions that increase water hardness. From lime-carrying minerals to calcium and magnesium ions, these pillows trap them all. This gradual process of removing ions and minerals also makes them safe for most fish.

Most water softening pad packs require you to attach them to your filter with hose clamps or zip ties. Some will need to be placed at the filter’s water inlet (between the hoses), while others will need to be placed inside an existing cartridge before being inserted into the filter. Pick one that works for you, your filter, and your tank!

See Also: Best External Filter Media Reviews

Use water softening crystals

If you want to soften your aquarium water on the cheap, try using water softening crystals. These products are salt mixes made especially for aquarium use, and many claim that they can soften the water up to 100% of the time. If your goal is a quick fix, this is one way to go.

This method works by replacing calcium and magnesium ions with sodium ions to lower GH levels. For example, if you have a lot of magnesium ions in your tank, the crystals will replace them with sodium ions, which «shifts» the ion balance. When the ion balance is changed, hardness minerals lose their power over how much they can bond with other ions in your aquarium water.

As with any chemical, care must be taken when using it to treat aquarium water. A general rule of thumb is to use one quart of water softening crystals for every 20 gallons of freshwater tank water. In marine aquariums, use half that amount or one liter for every 40 gallons of water. However, you should always read the dosage instructions carefully, as these specifications may vary depending on the product.

Remove existing tap water stains

If you have had tap water in an open container for days or weeks before using it for a water change, this increases the chance of calcium building up on the walls of the container. These accumulations are known as «water spots» and are known to be one of the most common causes of high GH levels in the aquarium. So it only makes sense to get rid of them before adding water to your tank.

Fortunately, this is easy and requires just a few products:

– CLR (Calcium, lime and rust remover)

– Hydrogen peroxide (3%)

The process works by adding all 3 parts together to make a chemical solution that attacks stains. Put equal parts of CLR, hydrogen peroxide and water in a container. If you don’t want to use the entire bottle of CLR or H2O2 in this project, go halfway. Gently shake the container every 5 minutes for at least 30 minutes to break down existing hard water stains.

Lower the pH of your water source

It may seem strange to start a discussion of pH when we are discussing ways to reduce aquarium GH, but it is a very effective way to remove existing minerals from your water source. The buffering capacity of water is directly related to its overall pH level, which means that the higher the pH of your water source, the more likely it is to retain existing minerals.

When you lower the pH of tap/well/source water with acidic buffers like gallic acid, you reduce its ability to bind hard minerals by neutralizing carbonates and bicarbonates. In other words, acidic water may retain less of everything, including the minerals calcium and magnesium. This means that your aquarium water will become softer as a result.

You can also opt for natural alternatives like Indian almond leaf. These leaves release tannic acid into the water, which systematically eats away at existing minerals. The only problem is that it can turn your aquarium water brown, causing some cosmetic issues. However, we believe this is a fair trade-off for softer water and healthier, happier fish.

Buy domestic water softeners

This method is by far the most expensive, but probably also one of the safest for aquarium inhabitants. These little machines are made to treat human drinking water, but the principle of operation is the same for a small aquarium. The machine passes your fresh or salt water source through a resin tank where minerals and carbonate ions are exchanged for sodium ions, reducing GH and KH levels.

This exchange process works continuously, so the only drawbacks are that you need a larger capacity tank for a decent volume of water. Also, the minerals that are removed are not added back to your aquarium. If this is a problem, you can always choose to remineralize your water with commercially available products.

When it comes to methods of lowering GH, this may be the most practical technique. All you have to do is pour the water into a bucket that goes into the machine and let it run through the system overnight. Although not a slow process by any means, you should still add this water to your aquarium in a slow trickle over a period of at least an hour to avoid shocking your fish.

Use inert, lime-free gravel

Using an inert, lime-free gravel is one of the best things you can do to lower the GH in your water. This is because lime-based gravels, such as limestone or dolomite, can rapidly accumulate magnesium and calcium ions over time. Those who wish to lower the GH level of their tank should also avoid substances such as gravel or crushed coral.

Limeless gravels are often marketed as «chemical-free,» which is great for fish farmers who want to avoid adding anything to the water. For example, cichlid keepers may already be familiar with cichlid litter, which is an inert material with a near-zero GH level. Other types of inert substrates include crushed granite, clay granules, or even river rocks.

In addition to selecting an inert substrate, you should also keep it clean. If you have an inferior substrate that is prone to trapping debris or food scraps, you risk raising your GH levels in the long run. For this reason, it is often used as a gravel vacuum to remove impurities that settle to the bottom.

Read: How much substrate do you need?

Introduce safe peat for aquariums

Many aquarium fish keepers often use peat moss to lower their pH, a process that is commonly known as «peat filtering.» However, some peat mosses on the market may also contain lime or dolomite rocks, which can lower your GH level after a period of time.

If you are going to introduce peat into your aquarium water, you must ensure that it is free of dolomite and lime. Fortunately, most peat moss sold as a growing medium is designed for planting aquatic plants in aquariums, which means it’s safe to use as a GH-reducing agent.

Peat works by binding to positively charged molecules like calcium or magnesium. When you add it to your aquarium and let it sit overnight, the peat falls out of suspension and falls onto the substrate below. Any excess peat can be removed in a gravel vac so it doesn’t build up over time.

How do we test the GH from the water?

Testing your aquarium water for GH is as simple as getting a test kit from your local fish store and adding a few drops to the water. These test kits are fairly inexpensive and allow you to measure your GH fairly accurately. If you’re looking for an even more accurate way to test your water’s GH level, choose a freshwater ichthyologist’s test kit, which can measure pH, KH, and GH.

To measure your aquarium’s GH, simply take a sample of aquarium water and add a few drops to the test solution. After waiting a few minutes, look at the color chart included with the test kit. If you’re looking for low GH (less than 60 ppm), look for a color closer to blue, and if your aquarium water has high GH (more than 70 ppm), look for a color closer to purple.

In general, GH is measured using parts per million or ppm. A reading of 0-59 ppm is considered soft water, 60-120 ppm is medium hardness, and anything above 120 is considered hard water. The ideal range for your tank would depend on the species of fish you have, but in general, you want to keep your GH between 4 and 12.

What is the relationship between GH, KH and pH?

The relationship between GH, KH and pH is quite simple. Essentially, the higher the levels of carbonate hardness (KH) and general hardness (GH) in your tank water, the more acidic the pH of your aquarium will be. This is because these minerals bind with excess hydrogen ions, reducing the amount of free hydrogen floating in the tank.

Simply put, GH and KH bond with positive hydrogen ions, reducing the amount of free hydrogen in your aquarium. If you have 100 ppm carbonate hardness (KH) and 200 ppm general hardness (GH), your pH would be 7 because there are fewer free hydrogen ions floating around. Soon:

  • The more GH and KH in your aquarium water, the higher its pH.
  • The less GH and KH in your aquarium water, the lower its pH.


We hope this article has provided you with information on how to reduce aquarium GH levels in your tank! Providing your fish with an ideal level of GH will ensure that they are happy and healthy, which is the most important thing when caring for an aquarium. In many cases this can make a big difference to the overall well-being of your fish!

Do you have any nagging questions about how to reduce water hardness for your aquarium? Feel free to ask us in the comments section below. We love to hear from our readers! And if you found this article useful, please do us a favor and share it with your friends. Your support means a lot to us, and we really appreciate it!

Thanks again for reading and happy fishing!

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