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    How To Soften Aquarium Water – 5 Safe & Effective Methods

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    How does hard water play a role in fishkeeping? Is it worth looking into how to soften aquarium water?

    You can soften aquarium water by reducing the concentration of dissolved calcium and magnesium. This process involves either deionization, chemical filtration, or peat moss/driftwood water softening.

    We’ll get into the most efficient ways to soften aquarium water in this article. But keep in mind that water hardness, like all tank parameters, isn’t an absolute value that you should chase.

    Most fish species bred and raised in captivity have a higher tolerance to harder water than their wild-caught counterparts. Keeping general hardness (GH), carbonate hardness (KH), and pH levels stable is more vital to the health of your pet fish than getting “ideal levels”.

    What Is Water Hardness?

    In fishkeeping, water hardness measures the concentration of dissolved minerals in aquarium water. 

    The minerals that water hardness testing focuses on are calcium, magnesium, and carbonate.

    The higher the concentration of minerals in aquarium water is, the harder the water is considered to be.

    The 2 parameters that water hardness testing strips use to determine water hardness are:

    1) General Hardness (GH)

    General hardness determines the concentration of dissolved calcium and magnesium in aquarium water.

    2) Carbonate Hardness (KH) or Alkalinity

    Carbonate hardness determines the concentration of carbonate and bicarbonate ions in aquarium water.  


    General Hardness (GH)

    Carbonate Hardness (KH)

    Units of measurement degrees of hardness (dGH)


    parts per million (ppm)

    degrees of carbonate hardness (dKH)


    parts per million (ppm)

    Equivalent of 1 degree 1 dGH = 10mg of calcium oxide per liter of water


    1 dGH = 17.84 ppm

    1 dKH = 17.86mg of calcium carbonate per liter of water


    1 dKH = 17.86 ppm 

    General water hardness can range between 10 mg/L to over 400 mg/L, depending on where the water is sampled. For reference, the Amazon River has a GH as low as 10mg/L, versus African rift lakes that can reach GH levels of 500 mg/L.

    The huge variations in water hardness between the native habitats of aquarium fish are why General Hardness is such an essential parameter to monitor.

    Pet fish are physiologically accustomed to the water conditions of their natural habitats. It’s up to us to replicate those conditions if we want them to thrive in captivity.

    Water hardness is a parameter that some soft water fish can tolerate, surviving in harder water than in the wild. But it’s close to impossible to breed and spawn soft water fish in hard water.

    Here are some good water hardness testing strips:

    How Does Water Become Hard?

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    Red discus (Symphysodon discus)

    Water streams in nature increase in hardness (GH) and alkalinity (KH) as water passes through rocks, like limestone or dolomite. Even limestone deposits in the soil can cause the concentration of dissolved minerals in water to rise.

    Depending on your region and the natural paths the water follows until it reaches your home, the hardness of tap water varies. As a result, it sometimes can be too hard to use “as is” in your aquarium.

    While the dissolved minerals in hard water don’t necessarily cause health problems in humans, fish are more vulnerable to these water quality parameters.

    When trying to determine the water hardness and alkalinity of your tap water, use these guidelines:

    Water Hardness

    General Hardness (GH)

    Alkalinity (KH)

    Very soft 0-4 dGH 0-4 dKH
    Soft 4-8 dGH 5-7 dKH
    Slightly hard 8-12 dGH 7-8 dKH
    Moderately Hard 12-18 dGH 9-12 dKH
    Hard 18-30 dGH 13-20 dKH
    Very hard >30 dGH >20 dKH


    How To Test Water Hardness In Aquarium Water

    Do you need to figure out how to soften aquarium water? A good starting point is to test the water hardness of your tap water “as is,” using aquarium test kits.

    Depending on the readings you get when testing your tap water, you can go 1 of 2 ways:

    • Research and stock your aquarium with fish species that are comfortable with the general hardness of the water you have on hand;


    • Use tried & tested methods to soften aquarium water if you’ve already stocked your tank with soft water fish. 

    How to test General Hardness (GH) in aquarium water

    To test the General Hardness (GH) of aquarium water, use a general water hardness test kit.

    This test will measure the concentration of dissolved calcium and magnesium in the water. 

    Lower values (0-8 dGH) indicate softer water, while higher readings (9-12+) indicate harder water. 

    How to test Alkalinity or Carbonate Hardness (KH) in aquarium water

    Alkalinity, or Carbonate Hardness (KH), can be tested using a carbonate hardness (KH) test kit

    The alkalinity of aquarium water is directly linked to its capacity to buffer fluctuations in pH levels. 

    Higher KH readings indicate the water has increased resistance to pH changes, which prevents instability in water quality. Conversely, lower KH values indicate that the water is susceptible to dangerous rapid changes in pH levels.

    If the Alkalinity level of your aquarium water tests below 4.5 dKH, you should definitely monitor pH levels more closely.

    Average water hardness values when testing stocked aquariums:

    Water conditions

    Freshwater Aquarium

    Saltwater Aquarium

    Alkalinity (Carbonate Hardness) 4 – 8 dKH 8 – 12 dKH
    General Hardness 4 – 8 dGH 8 – 12 dGH


    How To Soften Aquarium Water | Water Softening Methods

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    Guppies (Poecilia reticulata)

    Finding the perfect way to soften aquarium water to a level that’s comfortable for your pet fish can be tricky. Reminder: steady water parameters are way more important than completely altering the general hardness of tap water!

    A good starting point would be to look at a month’s worth of GH and KH readings on your test kits. Testing weekly should show you if there are significant fluctuations taking place in GH, KH, or pH levels.

    Softening tank water should typically be a slow and gradual process. This ensures that the fish won’t experience any dangerous side effects due to unstable water quality. 

    There are 5 ways you can safely soften aquarium water without putting your pet fish at risk:

    1) Rainwater

    The most inexpensive and effortless way to soften aquarium water is to perform all water changes using rainwater. Of course, you can always test collected rainwater to see what its GH and KH levels are, but it’s naturally soft.

    If you find that rainwater is too soft for your tank’s water conditions, you can mix it with tap water. However, there are a few safety measures you should take when collecting rainwater for your tank:

    • Make sure the container in which you’re collecting rainwater is as sterile as possible. 
    • Containers considered “food grade” are ideal since they won’t leach any chemicals.
    • Choose a rainwater harvesting method that ensures the water won’t get contaminated.

    If you’re looking to soften aquarium water using rainwater, you’ll need to also consider your location. 

    Industrial areas with poor air quality and high pollution levels aren’t ideal when collecting rainwater.

    2) Water Softening Pillows

    Water softening pillows are a type of chemical filtration media designed to lower aquarium water’s general hardness.

    The pillows contain ion exchange resins, which absorb calcium, magnesium, and other soluble heavy metal ions. This exchange process releases sodium ions as minerals get removed.

    This means that you can reuse water softening pillows. You simply soak the pillows in a brine (salt + water solution) for 2-4 hours to recharge them.

    Softening pillows, and other freshwater softening media, are efficient in lowering GH in smaller tanks (<30 gallons). However, using them as water softening media in a larger tank would be too time-consuming because you would need to recharge them every 48 hours.

    They are, however, an excellent choice if you’re planning on breeding soft water fish species, like Discus!

    3) Peat moss filters

    Peat moss, when used as filter media, reduces water hardness by binding calcium and magnesium ions. This process is called chelation, and it softens aquarium water through demineralization.

    In the process, peat also releases tannins and gallic acid. This exchange lowers KH and pH levels, as the acids neutralize carbonate and bicarbonate ions in the water.

    Before using peat moss as filter media, make sure you boil it for 2-3 minutes. This ensures that you won’t have to deal with hitchhiking parasites or other contaminants. You can also soak peat in clean water after boiling to prevent excessive yellowing of tank water.

    To soften aquarium water using peat moss, you can use one or more of these 3 methods:

    • Soak peat moss in a clean container to soften water

    If your tap water is too hard for the well-being of your fish, you can use peat moss softened water to level out General Hardness. The process should follow these steps:

    1. Boil peat moss to decontaminate it.
    2. Discard the brown water, and put the cooled peat moss in a clean container filled with tap water.
    3. Let the peat moss sit in the container for at least 2-3 days to soften the water.
    4. Use the softened water when doing a routine water change in your aquarium.
    5. Test GH, KH, and pH levels inside the tank to ensure there aren’t any significant fluctuations in water quality.
    6. Repeat as necessary.
    • Use peat moss in your filter as filter media

    Putting peat mass directly into your tank’s filtering system works great in softening aquarium water. 

    The constant flow of water running through the peat will boost its water softening efficiency.

    Here are the steps you need to follow:

    1. Wrap peat moss tightly using a mesh bag. Make sure the bag can fit into your tank’s filter.
    2. Boil the peat bag to remove contaminants.
    3. Allow the peat to cool in a container of clean water. Discard the water.
    4. Place the peat bag inside the filter behind a filtering pad.
    5. Test GH levels regularly to make sure your peat moss filter media has the expected effect.
    • Add peat moss into your tank as is 

    You can use peat moss “as is” in your fish tank to soften aquarium water. 

    You’ll still need to boil it, soak it to avoid brown water, and pack it into a filter bag. 

    Just make sure to secure the peat moss bag in an area of the tank that gets plenty of water circulation.

    4) Driftwood

    Driftwood works similarly to peat moss, softening the water by producing acids that neutralize carbonate compounds.

    The downside of using driftwood as a water softener is the tea-colored tinge that the water inevitably gets.

    It’s worth looking at a ‘pros vs. cons’ perspective when it comes to softening aquarium water using driftwood:



    Softens the water by releasing tannic acid. Will turn aquarium water brownish, at least for a short period of time.
    Tannins are harmless to fish. Can introduce parasites into the tank if not decontaminated well enough.
    Tannins can boost fish’ immune systems, protecting them against fungal infections. Needs to be monitored for fungal growth, especially if the tank isn’t particularly well aerated.

    5) Reverse Osmosis

    Reverse osmosis (RO) is a demineralization/deionization process used in fishkeeping to purify water. The process involved pushing any type of water through a semipermeable membrane, which captures up to 99% of impurities.

    This membrane also blocks large molecules, like minerals (calcium & magnesium), softening the water in the process.

    RO/DI system is an excellent investment if you own a larger capacity tank. However, if water hardness, or water quality, is a constant issue, reverse osmosis is the most reliable water softening solution.

    Here’s a comprehensive guide on RO systems and how you can use them to soften aquarium water:

    Why Would You Need To Soften Aquarium Water?

    Stabilizing water parameters, in an aquarium, at levels that are ideal for the fish species that you’re keeping is vital. Not only are fish adapted to have a certain set of water condition preferences, but some can simply not survive otherwise.

    Aquarium fish will leave little room for flexibility when it comes to parameters like salinity, pH, or nitrate levels. Levels outside the range of comfort for these water quality indicators will put fish in distress and even cause deaths.

    Luckily, water hardness is a tank parameter that most fish in captivity have learned to adapt to. Soft water captive-bred aquarium fish can survive in slightly hard to hard water.

    So why would you need to soften aquarium water?

    Allow soft water fish to thrive, rather than just survive

    Soft water fish species, like Angelfish, Barbs, and some Gouramis, can handle living in hard water community tanks. But fishkeepers know that there’s a big difference between fish surviving and fish thriving.

    With all other water parameters within healthy ranges and no signs of disease, soft water fish can still show signs of distress in hard water. Softening aquarium water will eliminate this stress factor, intensify their coloration and boost their immune systems.

    Breeding soft water fish

    Soft water fish can adapt to live in an aquarium where the water hardness is well above their ideal range. This doesn’t apply to breeding tanks when trying to encourage these fish to reproduce.

    Providing softer water that’s slightly more acidic and optimizing the temperature are essential steps in the process. Make sure you also acclimate your breeding pair before transferring the fish to a tank with drastically different water conditions.

    Prevent hard water stains and limescale deposits

    Cleaning hard water stains off of both acrylic and glass tanks can be a pain. And attempting to remove limescale build-ups can even leave behind scratches that will take away from the crystal-clear view.

    Soften aquarium water gradually to a water hardness range that works for your fish, and these aesthetic issues will solve themselves!

    Protect your fishkeeping equipment from frequent malfunctions

    You can use domestic water softeners to protect your home appliances from limescale build-up and the imminent frequent malfunctions. But these softeners do next to nothing for softening aquarium water, as they just replace limescale-carrying minerals with another set of minerals.

    When you soften aquarium water using any of the methods listed above, you’re also protecting your gear in the process. Filters, powerheads, and air pumps included!

    Soft Water Fish Species | Fish Species That Do Well In Soft Water

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    Ember tetra (Hyphessobrycon amandae)

    Soft water fish species are typically native to streams of water that travel over low-mineral soils. Wild-caught aquarium fish originating from South American, Southeast Asian, or West African rivers are perfect examples.

    Soft Water Fish

    General Hardness (GH)

    Ram Cichlids 3-6 dGH
    Discus 3-8 dGH
    Apistogramma Dwarf Cichlids 3-12 dGH
    Some Rasboras 4-12 dGH
    Some Gouramis 5-10 dGH
    Tetras 2-10 dGH
    Barbs 3-10 dGH
    Angelfish 3-8 dGH
    Cory Catfish 3-10 dGH


     Hard Water Fish Species | Fish Species That Do Well In Hard Water

    If you don’t have to worry about softening aquarium water, the most straightforward course of action is to stock your tank with hard water fish species. While some fish can survive in both soft water and hard water, these fish will thrive in hard to very hard water!

    Hard Water Fish

    General Hardness (GH)

    Guppy Fish 8-12 dGH
    Platy Fish 10-28 dGH
    Molly Fish 15-30 dGH
    Swordtail Fish 12-35 dGH
    Paradise Fish 5-30 dGH
    Archerfish 20-30 dGH
    Mono Fish 8-14 dGH



    how to reduce water hardness in aquariumPin

    Water hardness is a water quality parameter that you can either find ways to work with or end up constantly battling. Don’t need an extra to-do on your list? Just stock your tank with fish that do well in the type of water you have on tap.

    Do you have extremely hard water, or are you already keeping fish with a preference for softer water? There are ways to soften aquarium water, and you just need to choose one that works best for your setup.

    A Reverse Osmosis system is the best option for larger tanks and aquariums stocked with typically soft water fish.

    Keep in mind that monitoring GH and KH levels is crucial when you’re attempting to soften aquarium water. Aim for gradual changes!

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