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Water Chemistry (pH, GH and KH)

Discussion in 'Fish Keeping Chat' started by LinznMilly, May 13, 2020.

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  1. LinznMilly

    LinznMilly Moderator
    Staff Member

    Jun 24, 2011
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    Water isn't just water when it comes to fish. It was once said that keeping fish is more about looking after their water. And to a large degree, that's right.

    There are 2 elements to water that go hand in hand. The first has already been covered - ammonia, nitrite and nitrate, collectively known as water quality.

    The other element is water chemistry.

    Master test kits usually include pH, and some, such as the NT Labs now include GH (General Hardness) and KH (carbon at hardness) too. Some, like the API kit don't, meaning keepers have to buy them separately. You can get a ballpark figure of GH, and perhaps KH from the website of your water company, or by ringing the company up, but ballpark figures are just that - they can't tell you the exact values that are in your tank.

    So, why are they important?

    pH - Potential of Hydrogen
    If I was to say to you "Litmus Paper", chances are you'll know what I'm talking about. It's the scale along which fluids lie.

    On the one side, you have 4 to 6.9 - acidic
    On the other, you have 7.4 to 9 or 10 - alkaline
    7 is neutral.

    The NT Labs broad range pH kit,

    PH is important for a number of different reasons. Fish come from a fairly narrow band in which they sit on the pH scale. You may have heard that a stable but wrong pH is better than a right but unstable one, and you'd have heard right. PH swings stress and shock the fish, leading to a reduction in their immune systems and allowing pathogens to take hold more easily

    PH is also important because toxins (ammonia and nitrite) are more of a problem the higher the pH. They're also more toxic the higher the temperature, too.

    Finally, pH is important because it gives you an idea of the other 2 values mentioned in this post - GH and KH. If pH is above 7.6, chances are, you're in a hard water area. If pH is below 7, then there's a good chance you're dealing with soft water.

    GH (Geneal Hardness)

    This is the one that's perhaps most overlooked, and yet possibly more important than pH itself.

    Water is a soup of different minerals, especially magnesium and calcium, but others too. The higher the count of these dissolved minerals, the higher the GH, and the harder the water.

    Water Hardness scale

    dH ------------- | ppm or mg/l ------------ | Classification |
    0-3. -----|--------0-50. ---- ----------- | ------Soft. -------------------|
    3-6. -----| ------50-100 -- ----------- | ----Moderately Soft --|
    6-12 ----|------100-200 - ------------| --- Slightly Hard -------|
    12-18---|------200-300 --------------| --Moderately Hard ---|
    18-25 --| ---- 300-450 -- ------------| -----Hard -------------------|
    25+ -----| -----450+ -------- ------------| --Liquid rock -------------|​

    All fish need these minerals, electrolytes and salts, but the way they use them depends on their environment.

    Over millennia, fish have evolved and adapted to a fairly narrow range in water hardness. Generally speaking, fish from the Amazon and Asia are soft water species - these include a lot of the most common and easily recognisable fish in the hobby - Tetras, Rasboras, Barbs, South American cichlids, etc. Their natural habitats are often tannin-stained, giving the water a brown colour. The low nutrient value of these environments have caused the fish to evolve in a way that allows them to store the nutrients they need from their water - a bit like warm blooded animals regulation their body temperature.


    Fish from the Rift lakes in Africa, are generally hard water fish - the Old World cichlids - from lakes Malawi, Victoria, and Tanganyika. Livebearers such as Mollies, Platies, and Guppies are also hard water fish. The high number of dissolved minerals in the water means that the fish that live in these waters don't need to store the minerals in the same way that softwater fish do, therefore, they have a much closer relationship with their environment, and need their water to be within the hardness ranges they've adapted to. Take a guppy and put it in soft water and it'll be fine. For a short time, maybe weeks, maybe a couple of months. But soon it'll start to struggle, and if not corrected, it'll die. It will not thrive, breeding success rates will suffer and longevity will not be reached.

    Likewise, soft water fish also suffer, but their ailments are caused by an overload of nutrients - not too few. Again, breeding success is affected, and again the fish die prematurely.

    As above, it's taken millennia for the fisb to adapt to their surroundings - far longer than they've been kept in aquaria. Some aquarists will say they've "successfully" bred fish outside of their natural environments. That's one aquarist, with 1 aquarist's fish. The species as a whole have not been bred outside of their conditions, and therefore, there has been no adaptation to "alien" environments.

    KH (Carbonate Hardness)

    KH measures the water's ability to buffer pH and keep it stable. The higher the KH, the more stable the pH. Generally, it follows GH, so if GH is low, KH tends to be as low, if not a coupld of degrees lower.

    The Path of Least Resistance

    The easiest way to keep fish and help them thrive, is to choose species that have naturally adapted to the water conditions you can provide.

    Keeping Softwater Fish in a Hard Water area

    If you live in a Hard Water area, with pH above 7, but want to keep softwater fish, you'll have to bring the water hardness down. Adding pest moss to the filter, and going for a blackwater kind of look may help for a while, or if pH and hardness, aren't that high but a more reliable and permanent solution is to use RO (pure H2O - no dissolved minerals at all) and mix it with tap water. 50/50 RO/tap will roughly halve the GH and KH and reduce oH as a result. Or, if you want to get really technical, you can use RO and remineralise it.

    Keeping Hard Water in a softwater are

    This is actually easier to do than reducing hardness to suit softwater fish. Using aragonite sand, designed for Malawi cichlid tanks, will raise pH and hardness. Adding crushed coral to the filter will increase KH, and pH, but will not raise GH. Alternatively, once again, you can use RO water and reminereralise it to suit the requirements of the fish you intend to keep.
    #1 LinznMilly, May 13, 2020
    Last edited: May 14, 2020
    Magic Waves and magpie like this.
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