How to Soften Aquarium Water (5 Safe Methods)

Adjusting an aquarium’s water parameters if more often than not unnecessary. The majority of fish kept in the hobby are tank bred and can adapt to wide range of tank conditions if acclimated right.

Think twice before chasing numbers.

However, if you genuinely have a problem or trying to trigger spawning, this guide will cover everything you need to know on water hardness and how to soften aquarium water safely.

The Difference Between GH and KH

I’ve seen a lot of content online about reducing water hardness that is totally incorrect. So, I thought I should clarify some things.

There are two different kinds of “water hardness”:

  • General hardness (GH) – this is the amount of calcium and magnesium that is dissolved in water. This kind of hardness is what causes limescale, that white flaky stuff that builds up on your tank lids and glass.
    • GH is very important for fish osmoregulation, the movement of water and salts in and out of cells. The lower the GH, the more the surrounding water tries to enter a fish’s cells, and vice versa. This is why sudden, drastic changes in GH can be very dangerous for fish. 
    • General hardness DOES NOT have any effect on pH.
  • Carbonate hardness(KH) – this is the amount of carbonate and bicarbonate that is dissolved in water. KH has a direct effect on pH, the higher the KH, the higher the pH will be, and vice versa, the lower KH is, the lower pH is.
    • Sudden drastic changes in KH can also be very harmful to fish. Acclimate fish to new conditions slowly.

There are a lot of articles and videos that use GH and KH as if they are interchangeable but this is not the case!! 

GH, the amount of calcium and magnesium dissolved in the water, is what everyone classically refers to as water hardness. 

KH is the amount of carbonate and bicarbonate dissolved in the water. It measures the buffering capacity of the water, the amount of acid it can absorb before the pH begins to be affected.  

I often see steps to “reduce hardness” that advise adding some sort of acid to remove minerals from the water column, therefore “softening” the water. Adding acid to your water will eat up the carbonate and bicarbonate in the water, reducing the carbonate hardness (KH) and bringing down the pH.

But, adding acid to your water will not remove calcium and magnesium from the water, and will therefore have no effect on the general hardness of the water. 

So, filtering your water through peat moss, adding Indian almond leaves or adding an acid buffer will drop the KH and therefore the pH, and make your water more acidic, but it will leave the general hardness intact.

This is an important distinction that you need to understand before you start mucking around with your water chemistry: Are you trying to reduce the GH, the KH and pH, or both?  

How to Soften Your Aquarium Water

Here are five proven methods to soften aquarium water.

Method 1: Rainwater

  • Type of hardness: KH and GH
  • Lowers pH: Yes
  • Changes water color: No

This is exactly what it sounds like, capturing rainwater to use for water changes.

Rainwater is some of the softest water on Earth. It has almost no mineral content and usually has a pH around 5.0, depending on conditions.

By mixing rainwater with your tap water, you can lower the GH and KH of the replacement water you use to refill your aquarium. This is a good method if you’re trying to bring your pH below 7.0 since the rainwater’s pH is so low. 

Because conditions change throughout the year, rainwater may have different parameters at times. Make sure you do testing to verify the parameters of the water you put in the tank.

Testing your aquarium water is important. See our in-depth article about testing kits here.

The biggest pain about this method is collecting and storing the rainwater. You don’t want to use runoff that might be contaminated with pollutants. And you’ll need to store the water where bugs can’t get to it, or else your rainwater will become a breeding ground for mosquitoes.

You also have to wait for enough rain in order to do a water change. In some regions, this just isn’t realistic. You’d only be able to do a few water changes a year if you lived somewhere like Arizona.

But, if rain is plentiful in your area, rainwater is free and at least you don’t have to drive to the store to get it.


  • Free 
  • Reduces GH and KH when mixed with tap water
  • Good for bringing pH below 7.0


  • You have to collect and store rainwater
  • Not as exact as using distilled or RO/DI water
  • You’re dependent on rainfall for water changes

Method 2: Distilled Water

  • Type of hardness: KH and GH
  • Lowers pH: By itself, neutral pH of 7.0
  • Changes water color: No

You can buy distilled water, usually in gallon or 5 gallon increments, from your local grocery store.

Distilled water has been purified to the point that it is completely neutral, with a pH of 7.0 and zero mineral content.

Mixing in distilled water will lower the GH and KH of your tap water, diluting the mineral content. Over time, you should be able to figure out a basic mixing ratio that lets you zero in on the mineral content you want for your replacement water. Then, you can just follow that formula every time you do a water change.


  • Water is completely pure
  • You can figure out a reliable recipe for mixing 


  • You have to buy it/pick it up
  • Plastic containers have to be recycled

I personally use Salty Shrimp Mineral powder if I need to remineralize purified water for my fish tanks. Salty shrimp has a GH/KH formula and a GH only one as well. I’ve had great results with both of these products.

Preview Product
Salty Shrimp Bee Shrimp Mineral GH+ 230G Salty Shrimp Bee Shrimp Mineral GH+ 230G
SaltyShrimp - Shrimp Mineral GH/KH+ 200G SaltyShrimp - Shrimp Mineral GH/KH+ 200G

Last update on 2024-07-14 / Commissions Earned / Images from Amazon Product Advertising API

Method 3: Reverse Osmosis/Deionization (RO/DI)

  • Type of hardness: KH and GH
  • Lowers pH: By itself, neutral pH of 7.0 
  • Changes water color: No

Reverse osmosis filters force water through a super fine membrane that traps impurities and only lets pure water flow through. ​​RO/DI water has zero degrees general and carbonate hardness and a pH of 7.0, totally neutral. 

You can mix purified RO/DI water with your regular tap water to get the hardness levels you want. Once you figure out what ratios of RO/DI water to tap water gives you the right amount of dissolved minerals, you can follow that formula every time you do a water change.

This is much more exact than trying to mix unpredictable rain water. Plus, unlike rain water, there’s no need to store water for long periods.

But, there are some downsides to a RO/DI system.

The number one thing that turns people off is the cost. These units can be quite expensive, and the membrane needs to be changed out every so often, adding to the cost.  

Also, RO/DI systems are kind of wasteful. It depends on the brand, but most RO/DI systems produce 4 gallons of wastewater per 1 gallon of purified water.

RO/DI filters are also kind of slow. You’ll need to purify water in batches so you have enough for your water changes. If you have a big tank, this might be a realy pain.


  • Provides almost completely pure water 
  • Gives reliable results when mixed with tap water


  • RO/DI units are expensive
  • Wastes water, usually 4 gallons of waste per 1 gallon of purified water
  • Require installation and take up space under your cabinet
  • Takes time to purify the water in batches

If you only have a small tank (5 gallon (19 liter) or less), you can get a Zero Water filter for much less than a large reverse osmosis unit. The resin in a Zero Water filter removes over 99% of dissolved compounds in tap water.

ZeroWater 10-Cup Ready-Pour 5-Stage Water Filter Pitcher 0 TDS for Improved Tap Water Taste - IAPMO Certified to Reduce Lead, Chromium, and PFOA/PFOS
ZeroWater 10-Cup Ready-Pour 5-Stage Water Filter Pitcher 0 TDS for Improved Tap Water Taste - IAPMO Certified to Reduce Lead, Chromium, and PFOA/PFOS

    Last update on 2024-07-14 / Commissions Earned / Images from Amazon Product Advertising API

    Method 4: Indian Almond Leaves

    • Type of hardness: KH only
    • Lowers pH: Yes
    • Changes water color: Yes. Will leach brown tannins into the water.

    Indian almond leaves are harvested from the Terminalia catappa tree, native to the Indian subcontinent.

    These leaves leach out natural tannins, the same compounds that make brewed tea brown. Tannins are acidic, and so, they eat away the carbonate and bicarbonate in the water, therefore reducing the KH and lowering the pH.

    Tannins will stain the water brown. This will not harm your fish, but some people don’t like the slight tea-colored haze it can create in your tank. 

    Indian almond leaves are particularly great for shrimp tanks, especially Caridina shrimp. Not only do the tannins reduce the KH/pH, but the shrimp also eat the leaves.

    It’s hard to precisely control your water parameters using Indian almond leaves, because it is a natural product and there’s no way to know the tannin content from leaf to leaf. 

    Using Indian almond leaves is practical for tanks 20 gallons (76 liters) and under. For bigger tanks, I think it’s easier to filter the water through peat.

    These leaves would still be a great addition to larger tanks, especially for shrimp, but they’re not really a practical solution if you’re trying to reduce the KH of a large amount of water.


    • Natural product that reduces KH/pH slowly 
    • Shrimp eat the leaves
    • Doesn’t affect GH of water


    • Tannins can stain the water
    • Not really a way to precisely measure tannin content
    • Not very practical for large amounts of water

    You can let the tannins in your water eat up the KH and lower the pH, but then once the desired levels have been reached, you can filter out the brown color by running the water through activated charcoal. It’s a pain, but it is doable.

    Method 5: Peat 

    • Type of hardness: KH only
    • Lowers pH: Yes
    • Changes water color: Yes. Will leach brown tannins into the water. 

    Peat is a soil-like substance that forms from decaying layers of peat moss. Make sure that you get peat that doesn’t have any chemicals added to it. You want 100% pure peat.

    You can place it in mesh bags and put it into your aquarium filter. 

    Some aquarists filter their replacement water for water changes through peat. This usually entails letting water slowly drain through a container full of peat. I’ve seen a lot of different versions on the internet over the years.

    The peat very quickly leaches tannins into the water that eat up the KH and reduce the pH.

    The tannins from peat will stain your water brown. And just like Indian almond leaves, using peat to reduce the KH is not a precise endeavor. It will take some trial and error to get the levels you want.

    Peat has a similar effect to Indian almond leaves, but it’s easier to filter large amounts of water with peat, and Indian almond leaves are better suited to small amounts.


    • Natural product
    • Does not affect GH 
    • Better choice for large amounts of water


    • Not a precise process
    • Stains water brown

    All Aquarium Residents Need Minerals

    No matter what you read on the internet, absolutely all fish need some sort of dissolved mineral content in their water!!

    Even fish from the Amazon, like Discus or angelfish, or fish from blackwater Asian environments, like Bettas, need minerals in order to carry out their bodily functions. 

    Fish don’t get their minerals from the food they eat, they absorb them from the water they swim in. No minerals in the water means no access to minerals for aquarium inhabitants.

    And invertebrates like snails and shrimp rely heavily on the mineral content of the water. Lack of calcium can cause problems with molting and shell growth, often leading to death.

    So, you don’t want to just pour pure distilled or RO/DI water into your tank, it’s unhealthy for your livestock. That leaves you two options:

    • Mixing – Mix tap water with purified water to dilute the heavy mineral content.
    • Remineralizing – Use purified water only, but mix powdered minerals into replacement water to raise the GH and KH. 

    Do not use water that has been run through a home water softening unit in your aquarium. Water softeners put water in a state that cannot be remineralized, depriving livestock of vital nutrients. I recommend that you bypass the softener system in order to fill your aquarium.

    Stability is Better than “Perfect” Numbers

    You more than likely don’t need to mess with your water hardness.

    Don’t go changing your water chemistry just because you’re trying to make your water parameters match something you saw online.

    When writers, myself included, publish “ideal” numbers for fish species, those are based on the water conditions in that fish’s natural habitat.

    So, for instance, when I say that the “ideal” parameters for angelfish are a GH of 3-8, a KH of 3-5 and a pH of 6.0-7.0, that’s based on the general parameters of waterways in the Amazon River Basin. 

    But, the angelfish available in the aquarium trade have been bred in captivity for countless generations. People keep angels in a wide variety of aquariums all over the world that are nowhere near what the conditions are like in the Amazon. 

    Providing stable, clean conditions for your fish is often all you need to do to keep them happy and healthy. Most fish can acclimate to a variety of tank conditions. 

    It’s huge, sudden swings in water conditions that are harmful and stressful for fish, which is exactly what tends to happen when you muck about with your water chemistry.

    So, really consider whether or not you actually need to change your water conditions. Most often, you don’t.

    Exceptions to the Rule


    Ornamental shrimp, like cherry, bee or bamboo shrimp, prefer an environment that has a high GH. They utilize the calcium in the water to build their shells. 

    So, no need to mess with high GH tap water in order to keep shrimp.

    But, Caridina shrimp, like crystal shrimp, tigers, bee shrimp, etc, all need a super low KH and pH. Adding acidic compounds to the water will reduce the KH and bring the pH down, but leave the GH as it is. 

    I use Indian almond leaves in my own crystal red shrimp tank. The tannins they leach out are acidic, and the shrimp eat the leaves, so they serve a dual purpose.

    I highly recommend doing A LOT of research before bringing home any species of shrimp. Each species has specific requirements that need to be met


    One of the situations that does warrant messing with your water hardness is if you want your fish to spawn. Every species is different, so do some research on what your fish will need.

    Lots of fish cannot spawn without certain water conditions. You can put males and females together, but their bodies can’t go into reproductive mode without specific environmental triggers.

    For example, a lot of the species of tetras in the aquarium hobby need acidic water to start spawning. Likewise, some kinds of ornamental shrimp, like bee shrimp, won’t hatch unless the water is acidic. 

    You can get the adults to live and thrive, but if the water isn’t the right acidity, no babies.

    Aquarium Plants

    Some species of aquarium plants don’t like a lot of GH. So, if you’re trying to grow something that’s particularly picky, you may need to dilute your tap water to bring down the hardness.

    Final Thoughts On Softening Aquarium Water

    Changing your water parameters is often unnecessary to have a successful aquarium. Most species of fish can adjust to a wide variety of tank conditions if they are acclimated properly.

    I would advise you to think twice about why you’re trying to mess with your parameters. If your fish and shrimp are thriving and seem happy, leave it alone, you’re doing fine. 

    But, if you’re trying to trigger your fish or shrimp to spawn, or you’re having a hard time with a finicky plant, reducing your GH and KH may be necessary. 

    You can mix your tap water with purified water to dilute the mineral content. Or you can use 100% purified water and remineralize it. 

    No matter what method you use, just remember that aquarium inhabitants need at least some calcium and magnesium in the water. These are vital nutrients that all fish and invertebraes need in the water column.

    I hope you find this article helpful.

    I wish you and your fish the very best!

    Katherine Morgan
    Katherine Morgan

    Hey, there! I'm Katherine from Northwest Florida. An aquarium specialist, I've kept tanks for over two decades, enjoy experimenting with low-tech planted setups and an avid South American cichlid enthusiast.

    Leave a Reply

    Your email address will not be published. Required fields are marked *

    This site uses Akismet to reduce spam. Learn how your comment data is processed.