How to Reduce Water Hardness in Aquarium Efficiently?

When we address this topic, the two first concerns that may spring to mind are how to reduce water hardness in aquarium and why you should do so. To make things clearer, let me introduce to you a bit.

First, water is actually divided into 11 main kinds and hard or soft water are two of them. Hard water stands for the water including various mineral compounds while soft water is somehow opposite.

Although someone thinks every kind of water means the same things, hard water can kill your fish, make them stressed out and stunt the growth of various species. Therefore, it’s a must to lower water hardness in aquarium.

Ways to Reduce Water Hardness in Aquarium

Are you ready to fix hard water in fish tank with the 11 efficient methods below? Let’s scroll down for more!

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#1 Using Inert Lime-Free Gravel

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Simply use the inert lime-free gravel as a substrate of your tank to reduce water hardness effectively. How straightforward!

This type of gravel is chemical-free, which is beneficial to your pet fish. The lime-free substrate will assist you to prevent the mineral ions (such as calcium or magnesium) from accumulating with each other.

Therefore, this way will be best if you want to lower general hardness in fish tank (also called GH). Remember, make sure you do not use the pea, limestone, crushed coral or dolomite gravel.


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#2 Using Rainwater Or Distilled Water

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As rainwater and distilled water have pretty low GH levels, you can mix them with the hard tank water in order to decrease the mineral concentration. However, do not use merely rainwater to fill up your aquarium because your fish and plants still need some basic minerals to survive.

Therefore, you need to mix rainwater (or distilled water) with tap/remineralized water.


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#3 Doing Reverse Osmosis With A Specific System

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The only thing you need to do is install the reverse osmosis system into your aquarium and wait until the hard water turns soft again. Basically, this system “filters out” the contaminants by letting the water molecules go through a semipermeable membrane.[/su_box]

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#4 Using Driftwood

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First, boil the driftwood for 1 to 2 hours to sterilize it. Then, put the whole thing into your aquarium and ensure that it works well.

The principle of adjusting water hardness of the driftwood is to release acid tannic and brown the water. For your information, acid tannic doesn’t harm your pet fish so you do not need to worry about using driftwood for the tank.


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#5 Using Peat Moss

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Peat moss deals the best with problems caused by magnesium and calcium as it will trap them once there is water running through. In detail, the moderate amount of tannins included inside the peat moss aids in pH adjustment.

What to prepare:

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  • Peat moss
  • A mesh bag


Now, put the peat moss in the prepared mesh bag and drown it into the tank water. Peat moss can also become part of the substrate; assist you in controlling the ammonia level or act as a natural treatment for your tank.

However, if you wish to lower the hardness of aquarium water, which is overly serious already, this way is not a top choice.


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#6 Removing The Calcium Build-Up On Your Tank’s Walls

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Calcium will obviously “stick” to the walls of your aquarium on occasion. These mineral layers become “thicker” over time, resulting in a higher hardness level. Therefore, simply clear them off to get rid of hard water in fish tank.

What to prepare

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  • Calcium-Lime-Rust remover (CLR)
  • 3% hydrogen peroxide
  • Tap water
  • A container


Detailed steps

  • Mix Calcium-Lime-Rust remover, 3% hydrogen peroxide, and water with a ratio of 1:1:1 in a prepared container.
  • Gradually apply the mixture onto the calcium stains every 5 minutes.
  • Constantly repeat the action in the next 30 minutes and wait until the calcium walls fall off.


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#7 Using Acid Buffers To Decrease pH Level Of Your Water Source

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First things first, it’s necessary to choose an appropriate acid buffer and you can consider between gallic acid and Indian almond leaves. To use them, gently put the prepared thing into your tank and simply wait.

This method softens the tank water by decreasing the pH level, resulting in less magnesium or calcium in the tank water.

Indian almond leaves contain tannic acid (similar to the compounds included in driftwood) so your aquarium will turn brown. Even though it might make your aquarium less appealing, good thing takes time.

Also, here’s a guide that will help you find the right pH meter.


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#8 Adding Water-Softening Pillows To The Aquarium’s Filtration System

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Here is another way to lower GH in aquarium and it’s quite easy to do since you only need to throw the ready-to-use water-softening pillows into your tank’s filter. Otherwise, fixing it in the water inlet pipe will work as well.

In case you have no idea of which product to buy, refer to API Water Softener Pillow or something similar to it. It performs best in freshwater aquarium and still follows the GH-lowering principle of binding minerals (like magnesium or calcium).


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#9 Using Water-Softening Crystals

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Yes, these crystals look exactly like the things used in the regular washing machine. They also employ the idea of ion balance shift and gradually replacing the mineral ions with sodium ions.

Make sure you select good-quality water-softening crystals. Read the instructions on your chosen products, then, throw them into your tank

Notes: Only use ½ liter of crystals to 20-gallon of water (for marine/saltwater tank) or 1 liter of crystals to 20-gallon of water (for freshwater tank). However, the suggested amount can change based on the brands/types.


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#10 Using Domestic Water Softeners

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As you have a large container and a Domestic water softener, you can proceed with the following steps:

  • Fill the container with carbonate ions-decreased water and start to remineralize it.
  • Let the whole system continue working overnight.
  • Transfer the softened water into your fish tank little by little/drip by drip (to avoid shocking your dear pets).


  • Keep in mind that the replaced ions reduce general hardness (GH) and the carbonate hardness (KH) so well that you cannot directly use the filtered water in your tank.
  • To remineralize water, you can consider adding mineral salts, using mineral filters or alkaline water pitchers.


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#11 Frequently Doing The Water Change

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I guarantee that this technique is the least time-consuming way to lower hardness in fish tank.

You can prepare the following:

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  • Water suction machine
  • GH concentration test kit


What to do:

  • Test the GH level and see if it’s high or low.
  • Take out 25% of your tank water and replace it with the same amount of qualified water.
  • Check the GH level again to see whether it changes or not.
  • Redo all the steps above in the next few days until the water hardness level is now good for your fish.

Furthermore, use these aquarium water changers to do it easier.


Measure The Hardness Of The Water In The Fish Tank

It’s impossible to measure the GH or KH levels in your aquarium by looking only. You need to utilize the specific test kits (like multi test strips) to do so. After that, rely on this table and evaluate your tank’s problems.

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Water Hardness KH (in dKH/degrees of KH)KH (in ppm)GH (in dGH/degrees of GH)GH (in ppm)
Very Hard14+247+18-30320-530
Moderately Hard4-771-1258-12140-210
Very Soft0-20-350-40-70


Why is It a Must to Treat Hard Water in Fish Tank?


Once the fish tank’s water becomes harder, it will severely affect your fish’s health, physically and mentally. Some signs you can notice on your fish are:

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  • Being more aggressive (The changes in water force the fish to adapt to it quickly, leading to panicking and stress).
  • Unable to breed
  • Having various diseases or swimming idly (It’s because hard water weakens your pet’s immune system).
  • Becoming lethargic or experiencing sudden death due to osmotic shock


Because of these possibly unwanted results, you should decrease general hardness in aquarium (same as carbonate hardness) promptly. No matter whether you choose to soften aquarium water naturally or mechanically, refer to my previous instructions.



What is water hardness?

It’s generally defined by the concentration of magnesium/calcium ions. There might be some different kinds of minerals existing, but magnesium and calcium take the majority.

Besides using a water softener to remove hardness, you can also employ water conditioner lower hardness to condition the water.

What causes hardness in fish tank water?

Water hardness is caused by a variety of factors, the most common of which are water sources (hard tap water) or water running through limestone, soil (with limestone deposits), and dolomite.

How often should I test the water hardness?

I suggest doing it every 2 or 3 days if you are a novice because knowing when to lower GH and KH in aquarium is vital.

What fish can survive in hard water?

Thankfully, most freshwater livebearers (like swordtails, guppies, mollies, platies, etc.) can do well in hard water as they can adapt to the changes swiftly.


I must admit that you may find countless answers to your query of “how to reduce water hardness in aquarium” on the Internet; nevertheless, I believe this article is the most comprehensive.

You may pick and implement solutions ranging from ways to repair hard water naturally to more complex ones in real-life situations.

Please leave a comment or review in case you have used my methods before. I always welcome your constructive suggestions.