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  1. #41
    I did a small research project for a fellow instructor at Koi Organisation International on the subject of stabilising pH with Plaster of Paris blocks and concluded that it has become difficult to find “pure” plaster of Paris. Most of the available products contain curative additives that are questionably safe for koi.

  2. #42
    Senior Member Rank = Jussai Djstiles999's Avatar
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    II’ve never used it but was thinking it’d be useful in my grow on, would you advise against it based on the additives?

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  4. #43
    I've been sceptical about Plaster of Paris being effective at pegging the pH to any particular value because the way it adds carbonate is by adding limestone (calcium carbonate).

    Plaster of Paris is basically a mix of gypsum (calcium sulphate hemihydrate CaSO4.½H2O) which is a fine power that becomes hard when it’s wetted and allowed to dry plus some lime (calcium carbonate CaCO3) which is added to give it mechanical strength. There is often a curative ingredient added by the manufacturer to speed the hardening process and it’s these chemicals which I found to be questionably safe to add to a koi pond.

    The gypsum is calcium and sulphate which cannot do anything to the KH or pH but it will increase the GH.

    The lime is calcium carbonate and can add a source of carbonate to the water which will increase the pH but as I keep posting so often that I've copied and pasted this:

    Calcium carbonate is poorly soluble in water unless carbon dioxide is also present. If carbon dioxide is present then the calcium carbonate reacts with it to make calcium and bicarbonate both of which are soluble in water.

    We use sodium bicarbonate to raise KH (carbonate hardness) and it's the interplay between bicarbonate converting to and from carbonate that stabilises or buffers pH.

    There are many forms of calcium carbonate that you can put into a pond such as oyster shells, eggshells, limestone chips or powder, calcified algae etc.

    If you put any form of calcium carbonate into a koi pond and the pond is very well aerated, then there won't be a significant level of carbon dioxide. Without carbon dioxide being present the calcium carbonate won't dissolve to any great extent so not much bicarbonate will be produced and it will do little to stabilise the pH.

    If you put any form of calcium carbonate into a pond that isn't well aerated it will dissolve according to how much carbon dioxide is dissolved so some bicarbonate will be produced and that will have some effect on the stability of the pH.

    It all depends on how poorly aerated the pond is. If it's well aerated and there's only a small amount of carbon dioxide present there will be a small buffering effect. If the aeration is very poor and there's a lot of carbon dioxide present there will be a lot of bicarbonate produced so there will be a larger buffering effect.

    It isn’t true to say that calcium carbonate won't dissolve in a koi pond. What is true is that calcium carbonate is virtually insoluble in water unless there also is dissolved carbon dioxide present. This is the important statement to remember.

  5. #44
    Senior Member Rank = Yonsai jphamill's Avatar
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    Quote Originally Posted by Manky Sanke View Post
    I've been sceptical about Plaster of Paris being effective at pegging the pH to any particular value because the way it adds carbonate is by adding limestone (calcium carbonate).

    Plaster of Paris is basically a mix of gypsum (calcium sulphate hemihydrate CaSO4.½H2O) which is a fine power that becomes hard when it’s wetted and allowed to dry plus some lime (calcium carbonate CaCO3) which is added to give it mechanical strength. There is often a curative ingredient added by the manufacturer to speed the hardening process and it’s these chemicals which I found to be questionably safe to add to a koi pond.

    The gypsum is calcium and sulphate which cannot do anything to the KH or pH but it will increase the GH.

    The lime is calcium carbonate and can add a source of carbonate to the water which will increase the pH but as I keep posting so often that I've copied and pasted this:

    Calcium carbonate is poorly soluble in water unless carbon dioxide is also present. If carbon dioxide is present then the calcium carbonate reacts with it to make calcium and bicarbonate both of which are soluble in water.

    We use sodium bicarbonate to raise KH (carbonate hardness) and it's the interplay between bicarbonate converting to and from carbonate that stabilises or buffers pH.

    There are many forms of calcium carbonate that you can put into a pond such as oyster shells, eggshells, limestone chips or powder, calcified algae etc.

    If you put any form of calcium carbonate into a koi pond and the pond is very well aerated, then there won't be a significant level of carbon dioxide. Without carbon dioxide being present the calcium carbonate won't dissolve to any great extent so not much bicarbonate will be produced and it will do little to stabilise the pH.

    If you put any form of calcium carbonate into a pond that isn't well aerated it will dissolve according to how much carbon dioxide is dissolved so some bicarbonate will be produced and that will have some effect on the stability of the pH.

    It all depends on how poorly aerated the pond is. If it's well aerated and there's only a small amount of carbon dioxide present there will be a small buffering effect. If the aeration is very poor and there's a lot of carbon dioxide present there will be a lot of bicarbonate produced so there will be a larger buffering effect.

    It isn’t true to say that calcium carbonate won't dissolve in a koi pond. What is true is that calcium carbonate is virtually insoluble in water unless there also is dissolved carbon dioxide present. This is the important statement to remember.
    What a superb answer, many thanks! Illumination.

    Sent from my SM-G973F using Tapatalk
    2700 G Above ground, Nexus 220, EA 24 bead, 1m 2 tier custom Evolve shower 9kW Thermotech ASHP

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  7. #45
    Senior Member Rank = Jussai Djstiles999's Avatar
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    Appreciate the response, thought it sounded too good to be true, just keep adding the bicarb I guess. The point on CO2 is very interesting, on my pond having oyster shells in moving bed not going to be massively helpful then I guess

  8. #46
    Quote Originally Posted by Djstiles999 View Post
    Appreciate the response, thought it sounded too good to be true, just keep adding the bicarb I guess. The point on CO2 is very interesting, on my pond having oyster shells in moving bed not going to be massively helpful then I guess
    Yep, in those well used words, if it sounds too good to be true..........................

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  10. #47
    It all depends on how poorly aerated the pond is. If it's well aerated and there's only a small amount of carbon dioxide present there will be a small buffering effect. If the aeration is very poor and there's a lot of carbon dioxide present there will be a lot of bicarbonate produced so there will be a larger buffering effect.
    Maybe that is the key. I am running 17" diameter diffusers on each of the the two drains. Each diffuser is supplied by a Medo LA-120. And the shower has nearly a 100% turnover each hour. The pond is always near saturation for the water temp.

 

 
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