All about Water Hardness

Discussion in 'Water Chemistry' started by JamesC, 16 Feb 2009.

  1. JamesC

    JamesC Member

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    The topic of water hardness seems to often cause some confusion. This hopefully will clear up some of the confusion, and probably create some more.

    Most people measure their tap and tank water using two test kits, GH and KH.
    GH stands for General Hardness (Total Hardness) and is a measurement of total multivalent cations in the water.
    KH stands for Carbonate hardness (Temporary Hardness) and is a measurement of multivalent cations that are carbonates and bicarbonates.
    Both GH and KH are expressed in equivalents of CaCO3
    (A multivalent cation is a positively charged ion with two or more positive charges, ie, Ca++, Mg++, Al+++, Fe+++)

    What most people don’t realise is that the aquarium KH test kits don’t actually measure KH, but Alkalinity instead. Alkalinity has nothing to do with water hardness but is the buffering capacity of the water defined as the capability to neutralize H+ ions. This Alkalinity can come from many different compounds, but carbonate alkalinity tends to make up most of the alkalinity in our tanks. Other compounds that can contribute to alkalinity are phosphate, nitrate and silicates just to name a few.

    Coming back to water hardness. The following 3 types of hardness are the normal way of expressing water hardness.
    Total hardness (GH) is the total amount of multivalent cations in water and can be split into two parts, temporary and permanent.
    Temporary Hardness (KH) is the amount of multivalent cations that are carbonates and bicarbonates.
    Permanent Hardness is the amount of multivalent cations that are everything except carbonates and bicarbonates, ie. chlorides, sulphates, etc.

    Total Hardness = Temporary Hardness + Permanent Hardness

    Temporary Hardness (KH):
    To lower temporary hardness is fairly easily done. Boiling water will effectively remove it as the insoluble carbonates formed from the thermal breakdown of bicarbonates can be filtered off.
    To raise KH you can add calcium or magnesium bicarbonate. Adding baking soda (NaHCO3) will not raise KH as sodium isn't a multivalent cation, but will raise alkalinity as described above.

    Permanent Hardness:
    Lowering permanent hardness can either be done by diluting it with softer water, ie. rain water and RO water or by filtering it through a deioniser or RO system. Be aware that many domestic water softening ion exchangers replace calcium and magnesium ions with sodium ions. This type of water softener should not be used for planted tanks.
    Raising permanent hardness can be done by adding calcium or magnesium sulphate.

    Total hardness (GH):
    As GH is a combination of both temporary and permanent hardness, making any changes to these will affect the GH. So either boiling water or using a deioniser will lower GH. Adding any calcium or magnesium salt will raise GH.


    So what does all this mean for our planted tanks?

    GH or Total Hardness test kits are generally very good and do measure what they say they measure. GH is a good thing to have as plants need both calcium and magnesium present to thrive. If your water has a very low GH then this can be increased by adding GH booster or similar products, which normally contain calcium and magnesium sulphates. Most plants do fine in a wide range of GH’s so unless you have exceptionally hard water don’t worry about it.

    It’s KH test kits that are the problem as they are not measuring water hardness at all but alkalinity. This doesn’t really matter as we aren’t interested in KH anyway as alkalinity is much more important to us. Plants don’t require alkalinity to grow so you could keep your tank at zero, but this requires a lot of water changes to keep tank conditions stable as other alkalinity affecting compounds may build up. Some plants and fish don’t like very high levels of alkalinity so if you wish to keep these then the best way to lower it is by blending with reverse osmosis water. Fish are very sensitive to changes in alkalinity so any changes must be done slowly over a period of time.



    It is possible to lower the alkalinity by adding a strong acid such as hydrochloric acid as this adds H+ ions to the water. I have heard of some people adding strong acids to their tanks to lower the alkalinity but I highly recommend you don't try this if you have any livestock.
    Adding CO2 to the tank doesn't affect alkalinity (or KH test kits) at all. This is because equal amounts of H+ and HCO3- are formed so in affect cancelling themselves out.


    Hopefully this has been of some interest to you.

    James
     
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  2. aaronnorth

    aaronnorth Member

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    excellent write up, some info in there that i didnt know about. :)

    It might be worth adding that having a high kH does not affect the solubility of CO2 in aquariums (or at least not enough for us to notice any difference)

    Thanks.
     
  3. Nelson

    Nelson Member

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    hi
    i know this might sound dumb,but what is "exceptionally high gh".my tap water is 14-21dGH.

    thanks

    neil
     
  4. john starkey

    john starkey Member

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    Hi James,exellent article,well worth a sticky,regards john.
     
  5. JamesC

    JamesC Member

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    Those levels should be fine and are what Thames Water's levels are like. Only a few fussy plants like Tonina won't grow. Exceptionally high would probably be over 30dGH.

    James
     
  6. Antoni

    Antoni Member

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    Wonderfull article, James, but I can't understand how is possible adding CO2 not to affect the alcalinity or PH?Is it not the C02 that lower our PH when adding it to tank?
    I have been reading that the KH is buffer for PH - u need at least KH 4 to ensure, there will be no fluctuation of the alkalinity?
    Sorry, I dont argue with you, but just cant really understand how it works.
    Kind regards
     
  7. Nelson

    Nelson Member

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    thanks
    neil
     
  8. JamesC

    JamesC Member

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    When you add CO2 to water the following happens
    1) CO2 + H2O <=> H2CO3
    2) H2CO3 <=> H+ + HCO3-

    What is formed that affects alkalinity is H+ and HCO3-. As can be seen from the above equation equal amounts of of H+ and HCO3- are formed.

    Total Alkalinity = [HCO3-] + [OH-] - [H+]

    From the above equation you can see that if both H+ and HCO3- increase or decrease by equal amounts then the changes will cancel themselves out and the Total Alkalinity remains exactly the same.

    The pH changes because from the first equation above H2CO3 (carbonic acid) is formed which will lower the pH.

    Remember the KH test kits we use are actually alkalinity test kits. Alkalinity will always change when you add nitrates, phosphates, bicarbonates, bog wood, etc to the tank. The amount of alkalinity determines how much the pH changes. So if the alkalinity is high then it reacts with H+ ions keeping the pH up. This is known as buffering. But if the alkalinity is low then the ability to react with H+ ions is reduced and the pH will start to drop. This is why you so often see people recommending to keep the KH (alkalinity) at 4 or above. In practice though you can keep alkalinity at zero but you need to keep on top of water changes.

    James
     
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  9. Dave Spencer

    Dave Spencer Member

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    This an excellent write up James.

    I often read on other forums about peoples woes with a dropping KH and pH during cycling, when the reality has always been that it is the build up of nitrates etc. affecting the alkalinity.

    I may have to steal your succinct way of writing to show how injecting CO2 has no effect on the KH. :lol:

    Dave.
     
  10. plantbrain

    plantbrain Expert

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    Adding to this, KH test kits measure total alkalinity, not specifically bicarbonate (KH) alkalinity.

    They can add a few things to tap water to make this pH/Kh thing really screwy.
    Hard to say sometimes. It can throw off CO2 measurements by 10X however.


    Regards,
    Tom Barr
     
  11. GreenNeedle

    GreenNeedle Member

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    The CO2 not affecting KH is something that both I and Jack Middleton were trying to explain on TFF to a 'hardened' Ph Crash believer the other day when he was warbling on about CO2 lowers Ph and therefore lowers KH. We could not convince him of the fact that this was wrong.

    He just kept saying he had low KH of 6 and that if he adds CO2 his KH/Ph crashes so he doesn't add CO2.

    He then went on to say that due to his high KH he didn't really need to add CO" as his water naturally had 28ppm of CO2 in it before then adding he was not going to use TFF anymore saying (I only take the best bits out here:

    I am not going to posting on this forum any more, I am sick to death of the argumentative nature. I will go where my help is appreciated, and where I am a mod. I am fed up with seeing this all over the forum at so many posters that have supplied blooming good information. It may be the morphine talking but I really don't care anymore

    So the poster was implying that KH dropping from CO2, that 6 is too low a KH and that higher CO2 is naturally found in lower KH is seen by this poster as 'blooming good information'. He did also rave on about testing KH regularly too.

    Already I can see that I and Jack did get something wrong in that we said that adding Bicarb Soda increases KH whwere you say it raises alkalinity and not KH so whilst trying to get our point across we actually threw in something wrong ourselves :lol:

    I am now looking for KH6 water and/or morphine so I can sell my CO2 kit and have no CO2 worries :lol:

    Great work James

    AC
     
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  12. aaronnorth

    aaronnorth Member

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    I wondered why Jack PM'd me asking about the kH dropping with CO2 injection :lol:
     
  13. JamesC

    JamesC Member

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  14. LondonDragon

    LondonDragon Administrator Staff Member

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    Great write up James, I was actually looking for something on this a couple of days ago :)

    Which tests do you use or would recommend?

    Many thanks
     
  15. Dave Spencer

    Dave Spencer Member

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    I`ve had a few of these conversations recently, trying to convince people not to keep messing with their KH so they could inject CO2, yet they still insisted on doing it. Some people just love to worry, I think. My dKH is virtually zero, yet I can still inject CO2.

    pH crashes are all the rage on TFF at the moment.

    Dave.
     
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  16. Ed Seeley

    Ed Seeley Member

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    Has anyone actually had one? Or are they blaming their pH dropping on losing fish? Or are they just being paranoid because they don't understand what's going on?!
     
  17. GreenNeedle

    GreenNeedle Member

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    This will give your answer!! This one actually suggests that the Ph does crash rather than being afraid that it might happen

    A low KH makes pH more unstable.
    If I added CO2 to my tank with a KH of 6.0, my pH would crash, there is not enough KH to buffer the pH. That is where KH comes into this. Also the lower my pH goes consequently the KH does lower. If you have hard water you are lucky in this respect, damn lucky. I have to check the KH regularly before waterchanges and add Bicarb to the water going into the tank.


    AND

    Water with a low KH and low pH does tend to have alot more co2 content in the first place, that is why the CO2 chart is useful before you add co2. Just adding excel to my tank can cause instability problems.
    I don't need any extra co2 and plants grow pretty quickly in my tank.


    AND

    using the table I have
    KH 6.0 dch
    my pH is 6.8 therefore the CO2 level according to the chart is 28mg/l.
    If I added CO2 to that, I would poison my fish with CO2.


    AND

    We can all regurgitate information on pH from textbooks, but knowing how pH and KH affect your tank is the most important, this comes with experience. If you don't have ultra soft water and don't have to deal on a day to day basis with preventing pH and KH crashes, you will not understand how important checking KH is before adding CO2. The lower the pH in my tank the faster the KH drops, I have found this on numerous occasions, this is where watching experience is invaluable.

    I've been trying to advise on all these posts but getting more and more peeved sop tending to just ignore it now. Trouble is Jack bites and then posters spiel a load about experience and he gets stuck in a hole then pms me or Aaron. lol

    AC
     
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  18. aaronnorth

    aaronnorth Member

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    Most people are going on about pH crahses because they are fishless cycling, and when the are cycling the ph drops and if it gets below 5.5pH the cycle can stall, there must be 5 new posts about it everyday on there :rolleyes:
     
  19. Ed Seeley

    Ed Seeley Member

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    So nobody's had a pH crash, they just don't understand that pH will fluctuate more with lower KH! And to say they have enough CO2 because their pH is low! Oh dear! Best to stay out of it Andy, not worth the bother as they'll never change their minds!
     
  20. GreenNeedle

    GreenNeedle Member

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    But then they 'educate' the others with poor information which means there are now 2 people to spread the 'understanding' :) Like looking at a Queen Wasp and few workers building a nest over your back door and saying I'll leave it. :lol:

    2 months later it's Michael Caine time :wideyed:

    AC
     

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