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How To Lower GH In Your Aquarium? 6 Proven Methods

How To Lower GH In Your Aquarium? 6 Proven Methods

When I first started out in the hobby, I didn’t have the slightest idea about water hardness. I didn’t know what it meant, what it does, and why it was necessary. I’m sure a couple of my fish’s lives were cut short due to this. So, if you’re a beginner like I was once, I’ll teach you everything there’s to know on how to reduce GH in an aquarium. 

This is going to be a detailed guide. I’ll touch on various relevant and equally essential topics surrounding water hardness. 

So, buckle up! It’s going to be a long ride. 

How To Reduce General Hardness (GH) In An Aquarium? 

Reducing the water’s general hardness in an aquarium is indispensable for several reasons. The most common method is using natural elements like peat moss, driftwood, and rainwater. However, sometimes, we need to rely on other options like reverse osmosis and water softening pillows. 

Now, let’s have a detailed look at them!

Peat Moss Filters 

Peat moss is my preferred choice for lowering the general hardness in an aquarium. It helps to decrease the hardness by binding magnesium and calcium ions. This process is known as chelation. 

In addition to demineralizing the water, peat moss also releases certain tannins and gallic acid in the water. And this exchange helps to lower both KH and pH levels since the acid neutralizes the water’s bicarbonate and carbonate ions. 

Peat moss is readily available for purchase both online and offline. But some preparation is necessary before you use it.

Peat moss may contain contaminants and pathogens that can turn the water brown. So, boil it vigorously until the moss releases all brown colors. 

If time is on your side, you can simply soak peat moss in water for 3-4 days. 

There are basically 3 ways to use peat moss to soften aquarium water. 

They are:

  • Using in an extra outside container 
  • Using inside the aquarium filter
  • Using as a substrate layer 

Using in an extra outside container 

This is the most straightforward way of using peat moss which involves softening a certain amount of tap water before pouring it into the tank. But note that you still have to boil or soak the moss before using it. 

For this technique, you will need small-sized mesh bags. Add the peat mosh into the mesh bags as tightly as possible. Next, place these bags in a container filled with water. 

Test the DGH and pH value routinely – I’d do once every 10 hours – until the hardness lowers to your desired value. 

Remember, you cannot go overboard with this method. The sudden and drastic change in the environment will do more harm to your fish than wrong water parameters. 

You can add softened water every week in established tanks while changing water. 

Using inside the aquarium filter 

This one is an easy but super-effective way of softening tank water. Simply insert a peat moss bag inside the filter to be used as filter media. The prepping process for moss is the same as I explained above. 

If you have boiled the peat moss, make sure it fully cools down before adding it inside your filter. You don’t want to mess with the water temperature.

I have read that placing the moss bags between the charcoal media and filter pad allows more effectiveness and better results. 

Using as a substrate layer 

This method requires a bit more elbow grease than the above two. However, you won’t need to tightly stuff peat moss into mesh bags. Instead, you can spread the moss across the tank’s bottom and cover it with another layer of substrate – sand or gravel. 

That’s because peat moss is light and fluffy. It can be effortlessly relocated by the water’s movement and inquisitive fish. The additional layer of gravel or sand also helps prevent the formation of anaerobic pockets that encourage the growth of harmful bacteria. 

However, this method has one downside, which makes me choose the aforementioned two above this. Since the substrate absorbs minerals, it will brown the water even though you have boiled and cleaned the moss beforehand. 

If brown isn’t really your aesthetic, you can skip this method. 

Pros Of Using Peat Moss 

  • Natural water softener 
  • An economic option 
  • Easy to use and effective

Cons Of Using Peat Moss

  • The water may turn brown 
  • Not very effective if water has a high GH value 

Reverse Osmosis (RO/DI)

If your tank’s water hardness increases more often, you might want to invest in a Reverse Osmosis (RO/DI unit). You can improvise all you want with DIY methods, but getting a RO/DI unit will save you a lot of time and bucks in the long run. 

This system passes the water through a membrane filtration unit to deliver 100% pure water. However, since RO water contains absolutely zero hardness, which isn’t desirable for aquariums, you can partially mix tap water with it or remineralize RO water by adding minerals. 

An RO/DI unit may cost you more upfront, but it will be profitable in the long run. If you have a large capacity tank, we suggest getting your hands on one of these. 

Make sure to watch this in-depth video on how you can use the RO system to reduce water hardness in an aquarium.

Pros Of Using RO/DI Units 

  • A permanent solution
  • An economical option in the long run 
  • Provides zero hardness 

Cons Of Using RO/DI Units

  • High upfront cost 
  • Bulky built


With driftwood, you can kill two birds with one stone! First, it helps to lower the water’s hardness. Second, it lends an absolutely stunning look to your tank. 

Driftwood works its magic the same way as peat moss. It produces acids and tannins that neutralize the carbonate compounds present in the tank. 

Not just that, driftwood is also used to lower the water’s pH levels to create a slightly acidic environment that most prefer. And let’s not forget how it helps promote the growth of beneficial bacteria in the tank! 

Malaysian driftwood works the best since it has a very high tannin concentration. If you’re not a fan of driftwoods, you can use tannin-producing leaves like the Indian almond leaves. 

Driftwood can be readily bought from stores, or you can collect them yourself from nature. I’d go with the first option since manufacturers already cure and prep it to make it safer for aquariums. 

However, it doesn’t mean you can place it straight into the tank right after purchase. You will still have to clean it thoroughly, wash, sterilize, and cure it before it can safely go inside the tank. 

The potency of releasing tannins differs from driftwood to driftwood. Some release tannins even after six months, while others work only for a month or two. 

So, if you notice the water’s hardness increases even with the driftwood inside it, you might want to replace it. 

If you’re interested to know how to pretreat the driftwood to make it safe for your fish, you might be interested in this article

Pros Of Using Driftwoods 

  • Reduces water hardness gradually 
  • A natural and safe method 
  • Tannins boost fish’s immunity 
  • Prevents fungal infections

Cons Of Using Driftwoods 

  • Preparation is a tricky and lengthy process 
  • Water will turn brown 
  • Can leach parasite into the tank if not treated properly

Water Softening Pillows 

Water softening pillows are readily available chemical filtration media that reduce the water’s general hardness. These pillows contain ion exchange resins that replace calcium, magnesium, and other water-soluble heavy metal ions. 

So, once again, you can say these work by lowering the calcium and magnesium concentration of the water. 

What I love the most about these ion exchange resins is that they don’t harm your fish and aquatic plants in any way. These pillows are also known to eliminate the white crusty deposits that often plague the tank’s surfaces. 

Since these pillows lose effectiveness over time, you will have to recharge them routinely. But recharging is a breeze. 

You simply have to soak the pillows in salt and water solution for 2-4 hours. 

These work the best for small tanks under 30 gallons. For big ones, you might want to look into other options. Otherwise, you will end up recharging the pillows every third day. 

Pros Of Using Water Softening Pillows 

  • Rechargeable and reusable
  • Doesn’t cost much 
  • Can be used as filter media 
  • Effective for small tanks 

Cons Of Using Water Softening Pillows 

  • Recharging can be tedious 
  • Not a natural method
  • Not suitable for big tanks 


Using rainwater is hands down the most natural way of lowering your tank’s GH values. Needless to say, it’s also the most inexpensive method. You can always test the rainwater’s GH and KH values, but it’s generally always soft.

However, not all areas are blessed with rainwater throughout the year. So, if you could make an arrangement to collect the rainwater to be used on dry days, I’d recommend that. 

For example, it can simply be a large and clean container with a lid under the blue sky. You can take off the lid and let the water in whenever the rain falls. 

If the rainwater in your area is too soft to be used in a fish tank, you can always mix it with some tap water. 

Here are a few steps to ensure the rainwater you use is as safe as possible:

The container you use to collect rainwater should be clean and sterile at all times. 

It’d be best if the container is labeled ‘food grade’ so it doesn’t leach chemicals into the water.

Opt for a rainwater harvesting method that doesn’t contaminate the water.

Skip this method entirely if you live near industrial areas or cities with high pollutants levels. 

Pros Of Using Rainwater 

  • Inexpensive method 
  • 100% natural yet effective method
  • Sustainable approach 

Cons Of Using Rainwater

  • Requires comparatively more effort 
  • Not feasible in all areas 
  • Not practical year-round 

Distilled Water 

Using distilled water is the last on the list but not in terms of effectiveness by any means. You can buy distilled water that’s readily available for purchase or prepare your own at home if you feel like using some elbow grease. 

The water is purified via a distillation process. It is first boiled and turned to vapor. The steam is then collected and condensed to collect distilled water. 

As you have already guessed by now, distilled water is completely free of minerals. The vapor leaves everything behind during evaporation, leaving you with pure water. 

However, you should still dilute distilled water with some rainwater until you reach desired GH levels before pouring it into the tank. 

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What Is Water Hardness?

Chemistry doesn’t always have to be full of jargon, right? Water hardness measures the concentration of dissolved minerals in the water. The minerals mainly responsible for making the water hard are magnesium, calcium, and carbonate. 

So naturally, the higher the mineral concentration in the water, the harder it will be. 

What’s The Difference Between General Hardness And Carbonate Hardness?

General hardness (GH) measures the concentration of calcium and magnesium ions in the water, whereas carbonate hardness (KH) measures the concentration of free carbonate and bicarbonate ions. 

Usually, we use carbonate hardness to measure the water’s alkalinity – hence, it’s directly related to pH. So, when we say water hardness, it actually refers to just general hardness. Don’t get confused! 

Here’s a table that will make things more clear:

General Hardness (GH)
Carbonate Hardness (KH)
Units of measurement
Degrees of hardness (dGH) or parts per million (ppm) Degrees of carbonate hardness (dKH) or parts per million (ppm)
Equivalent of 1 degree
1 dGH = 10mg of calcium oxide per liter of water or1 dGH =
17.84 ppm
1 dKH = 17.86mg of calcium carbonate per liter of water or 1 dKH = 17.86 ppm 

Even though they’re born and raised in captivity, pet fish are physiologically accustomed to the water parameters of their natural habitats. If they’re exposed to the wrong environment, their fragile lives will be cut short abruptly. 

The general hardness can range from 10 mg/l to 500 mg/l depending on where the water comes from. 

To give you an example, American cichlids like angelfish and oscars come from Amazonian river basins where the water’s GH value is as low as 10mg/L. 

On the other hand, African cichlids like mbunas and peacocks come from rift lakes where the GH value can go as high as 500 mg/l. 

So you see how important it is to get the water’s GH values right. 

Here are two tables that will give you an idea of how hard the water should be for different tank setups. 

Water HardnessGeneral Hardness (GH)
Very Soft0-4 dGH
Soft4-8 dGH
Slightly Hard8-12 dGH
Moderately Hard12-18 dGH
Hard 18-30 dGH
Very Hard>30 dGH

Tank SetupGeneral Hardness (GH)
Tropical Fish Tank4-11 dGH
Cichlid Tank12-20 dGH
Shrimp Tank4-8 dGH
Marine Tank8-12 dGH
Brackish Tank12-20 dGH
Axolotl Tank7-14 dGH
Fish Pond 4-11 dGH
Planted Tank3-8 dGH

How Does Water Become Hard?

Water in nature passes through several rocks like limestone and dolomite. There’s some extent of mineral concentration in soil too. So, when the water passes through these surfaces, the minerals gradually leach into the water – causing the concentration of dissolved minerals in water to rise. 

The hardness of tap water will vary based on the location and the natural path that water follows before it reaches your area. And while these minerals may not pose any health hazards for mammals, fish are highly reactive to them. 

How Is General Hardness Measured? 

Thanks to advancements in technology, you can measure the general hardness of your tank’s water from the comfort of your home. Several ready-to-use test kits are readily available to choose from. 

We use this API Test Kit that measures both general and carbonate hardness. 

We stick with liquid-based tests instead of strips because they’re more reliable. 

Why Should You Soften Aquarium Water?

You don’t necessarily have to always soften aquarium water. Some fish thrive in hard water. However, if you have species that need soft water, you shouldn’t expose them to wrong water parameters. In soft waters, they won’t just survive and thrive but also breed readily. Also, it helps to avoid hard water stains and limestone deposits, as well as protects your equipment from rusting and frequent malfunctions. 

Allows Fish To Thrive 

Certain fish species like angelfish, oscars, gouramis, and barbs need soft water to thrive. To put it simply, their systems aren’t built to intake all the minerals present in the water. 

Therefore, if these fish are consistently exposed to hard water, they will be highly stressed. The stress, in turn, will weaken their immunity, stunt their growth, and make them vulnerable to opportunistic pathogens present in the tank that are waiting for the right opportunity. 

Encourages Breeding 

Even if your soft water species survive in hard water conditions on a wing and a prayer, they will simply not breed. Fish will go with their instinct and refuse breeding in unfavorable conditions. 

To encourage soft water species to breed, you should lower the tank’s general hardness, increase acidity slightly, and optimize the temperature. You can also conduct a big water change and feed protein-rich food to strengthen the breeding chances. 

Also, if you are transferring the breeding pair to a separate tank, make sure to acclimatize them to the new environment gradually to avoid stress and shock. 

Prevents Hard Water Stains And Limescale Deposits 

Removing hard water stains from the tank is painstakingly difficult, whether you have an acrylic or a glass tank. On top of that, when trying to get rid of the limescale buildups, you will end up with unsightly scratches that obscure the tank’s view. 

You won’t face these problems with soft water. I rest my case! 

Protects Equipment From Malfunctioning 

Hard water makes your tank equipment prone to limescale buildup and corrosion – resulting in their frequent malfunctioning and short lifespan. Whether it’s powerheads, filters, or air pumps – everyone suffers when continuously exposed to hard water. 

So, without a shadow of a doubt, soft water ensures proper functioning and extended lifespan of various tank equipment. 

However, not all fish are built the same. While some thrive, others suffer in soft water. 

Let’s look at fish that thrive in soft and hard water, respectively. 

Fish That Thrive In Soft Water 

Cory catfish3-10 dGH
Angelfish3-8 dGH
Tetras2-10 dGH
Gouramis (some)5-10 dGH
Rasboras (some)4-12 dGH
Ram Cichlids3-6 dGH
Discus3-8 dGH
Apistogramma dwarf cichlids3-12 dGH

Fish That Thrive In Hard Water

African cichlids11-22 dGH
Guppies8-12 dGH 
Mono fish8-14 dGH
Archerfish20-30 dGH
Swordtails12-35 dGH
Mollies15-30 dGH
Platies10-28 dGH
Paradise fish5-30 dGH

Final Words: How To Lower GH In Your Aquarium? 

Water’s general hardness measures the concentration of magnesium and calcium ions in the water. While some fish like guppies and cichlids thrive in hard water, others rely on soft water for their survival. 

You can lower the GH in your aquarium quite easily by yourself. The most common method is using peat moss filters. That’s what I prefer using too. 

Other options include reverse osmosis, distilled water, rainwater, driftwood, and water softening pillows. 

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