Question Of The Day: How To Lower GH In An Aquarium?

Last Updated on December 7, 2021 by Marco C.

You have discovered that water quality is not exactly the best, and you are now wondering how to lower GH in an aquarium? Find out more!

General hardness, commonly known as water hardness is simply a measure of the concentration of naturally occurring salts that have been dissolved in water.

It’s an important water parameter for healthy saltwater and freshwater fish tanks. However, many aquarium hobbyists don’t pay attention to it. If fish are kept outside their optimal hardness range, it could cause them not to grow as big or not live as long.

You have noticed that your general hardness has exceeded the allowable limit. Now you wonder how to lower GH in an aquarium? Keep reading to find out the easiest ways to solve this problem.

What Is GH In Aquarium?

General Hardness (abbreviated GH) measures the amount of calcium and magnesium ions in the water. That is, it tells us about how hard or soft your water is. This ultimately means, the harder the water = the higher the GH.

The GH is measured in degrees (one degree is equal to about 17.9mg/l). When written, the degree symbol is often replaced by “d” (i.e., 6dGH).

In addition, GH also shows if water has enough salt and minerals needed for healthy biological functions. Some of them are the development of muscles and bones of fish, the development of shells, and the growth of plants.

What Should The GH Level Be In A Fish Tank?

Water with low GH is said to be soft, and on the other hand, water with high GH is considered hard. In other words, soft water has very little or no calcium and magnesium, in contrast, hard water abounds in it.

Hard/soft classification of water

0–4 dGH – very soft

4–8 dGH – soft

8–18 dGH – medium-hard

18–30 dGH – hard

30 + dGH – very hard

Click here to find out more Hard Water information.

The freshwater aquariums usually have a GH between 4-8 dGH. Although all fish need minerals, some species (livebearers, goldfish, and African cichlids) prefer higher levels of GH.

How To Reduce General Hardness In Aquarium

We have come to the most important part, these are the best methods on how to how to lower GH in an aquarium.

Reverse Osmosis (RO)

Reverse osmosis is a process of water purification in which pressure pushes water through a semi-permeable membrane covered with tiny pores. This process eliminates about 95 to 99 percent of minerals, chemicals, and other substances, and the water becomes clear and pure.

Note that the RO process is quite slow, so it would be good to split it into a few smaller bits. Also, RO water should be mixed with tap water to maintain a balance between softness and hardness. This will eventually lead to a balanced pH level of the aquarium.

Applying reverse osmosis is one of the most effective ways to lower GH in your aquarium.

Water Softening Pillows

Water Softening Pillows effectively reduce GH by lowering calcium and magnesium. They use some kind of chemical filtration medium (ionized resin) to reduce the hardness of the water. This will reduce the level of magnesium and calcium, and increase the level of sodium in the water of your aquarium.

However, as much as they lower the GH level, they simultaneously increase the sodium level in the aquarium. This can be devastating for your aquatic pets, therefore, pay attention to it.

WSP can be re-used and re-charged several times by immersing it in a salt solution for a while. They are designed for smaller tanks but will work well in larger ones as well. The only difference is that you will have to recharge it several times more.

Peat Moss & Acid Release

As for reducing carbonate hardness (KH), peat moss can be used to reduce general hardness in the aquarium as well.

Peat binds magnesium and calcium ions, releasing tannic and gallic acid. These two released acids attack and break down the bicarbonates in the water. This ultimately results in a decrease in carbonate levels as well as pH.

You may notice over time that the water in your aquarium has taken on a brownish color, but don’t worry. It’s just the tannin that your peat releases. It’s completely harmless and will soon disappear.

You can add peat moss in one of the following three ways.

  1. Installation inside the tank filter.
  2. Pre-add to a bucket of water at least two weeks before use.
  3. Its addition to the pillowcase and direct immersion in the aquarium.


Using driftwood is another powerful way to lower GH in an aquarium. It works on a similar principle as moss. It would be good if you could buy it online or at a local fish store. This way you will avoid introducing invasive species into your aquarium. Even if you buy it, try to boil it first to make sure it’s hazard-free.

Similar to moss, there is a chance that your aquarium water will take on brown notes. These are also tannins released by driftwood. If you want to avoid this, soak the driftwood for a few weeks before use.


Read more about: Freshwater Aquarium Rocks and Driftwood

Make sure your water is moving and that it is properly aerated. Also, pay attention to whether slimy algae or similar growths form on the wood. Regular checkups can save your aquarium from fatal consequences.


Using rainwater is one of the best low-budget options ever. Keep in mind, not all rainwater is good. Therefore, collect it only in places with good air quality to ensure that your rainwater is chemical-free.

Also, rainwater is very soft, so it will most likely need to be mixed with tap water or special aquarium salts to raise GH and KH levels to appropriate levels.


Water quality is of great importance for the overall health of your aquarium. High levels of GH in your water can stress your fish or worse, they can even result in fatal consequences.

Therefore, regularly conduct aquarium checkups to provide your fishes with all the necessary conditions for a long and happy life.

Do you have more questions about how to lower GH in aquariums? Write below!

Read next: How To Reduce pH In An Aquarium