# Which filter can reduce KH?



## Hanuman (21 Nov 2019)

Hello all,

I would like to know if there is a filter (other than a complete RO system) that can reduce the KH for the purpose of using the water in a tank? I want to acquire some sensitive plants that require a KH<3 and want a way to reduce KH permanently.

Perhaps Ion exchange filters? Cation exchange? I know those don’t remove the minerals per say and rather swap them but are they suitable/effective for reducing KH? Yes? No? Why?

Thank you everyone for enlightening me.


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## Thumper (21 Nov 2019)

Hello,

cation exchange isn't suitable for in tank use due to the raising CO2 levels which have to gas off. You could go with some acids to remove the KH, but this requires exact knowledge of your KH and how much you would want to remove - and then again different reactions cause difference parameters to raise.


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## dw1305 (21 Nov 2019)

Hi all,





Hanuman said:


> I would like to know if there is a filter (other than a complete RO system)


Not easily. Small volumes of water you can boil (carbonate content is dependent on the level of dissolved CO2), but then you need to strain the boiled water through a fine membrane (coffee filter) to remove the precipitated CaCO3. As the water cools CO2 from the atmosphere will diffuse back in and  the dissolved CaCO3 will go back into solution as Ca++ and 2HCO3- ions. 





Hanuman said:


> Perhaps Ion exchange filters? Cation exchange?


It would need to be anion exchange resin to reduce dKH. You can get them for tap water filters etc.





Hanuman said:


> can reduce the KH for the purpose of using the water in a tank


You can add an acid, that converts the HCO3- ions to CO2, and the CO2 will then outgas. As you know I'm not keen on messing about with strong acids. Seachem sell an acid "buffer", this is just a mixture of acids.

It is much easier to start with a low dKH water source, is rain-water an option?

cheers Darrel


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## Hanuman (21 Nov 2019)

Thanks Darrel.



dw1305 said:


> It would need to be anion exchange resin to reduce dKH. You can get them for tap water filters etc.


This would be the best bet then. But isn't sodium exchanged in the process?



dw1305 said:


> You can add an acid, that converts the HCO3- ions to CO2, and the CO2 will then outgas. As you know I'm not keen on messing about with strong acids. Seachem sell an acid "buffer", this is just a mixture of acids.



Yes I rather not go down the acid route as it complicate things.



dw1305 said:


> It is much easier to start with a low dKH water source, is rain-water an option?


I would never put rain water in my tank here in Thailand/Bangkok. That water comes with all sorts of nasty things once it washes the overly polluted air. Plus it's the dry season now so no. I simply wanted to know a simple and effective way to drop dKH straight from tap water without loosing too much pressure. I do have an RO filter but it's mostly used for drinking and my smaller tank where I use a 50% tap - 50% RO. That brings dKH to ~3 which is fine. For my bigger tank I don't see myself pouring 50L of RO water in it every week. It's already a pain to change water with buckets.


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## MJQMJQ (21 Nov 2019)

Chemical filters might work or media eg activated charcoal.Will need replacement every few months and Im not sure how much it can remove.


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## Thumper (21 Nov 2019)

MJQMJQ said:


> eg activated charcoa


Nope, thats not gonna work.


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## Hanuman (21 Nov 2019)

MJQMJQ said:


> Chemical filters might work or media eg activated charcoal.Will need replacement every few months and Im not sure how much it can remove.


As mentioned above, I am not into adding acids to my tank water. I am looking for a filter. As for activated carbon, it will not drop dKH as far as I am aware.


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## MJQMJQ (21 Nov 2019)

Actually ure right haha even if the substances that increase KH are captured by the charcoal they still can react. Buying distilled water may work meanwhile but evaporation is stupid.Tank covers will reduce evaporation.


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## ian_m (21 Nov 2019)

You can use ion exchange resins to swap out the calcium and magnesium for potassium. Just use a standard ion exchange water softener or you can get ion exchange resin in "pillows" for aquatic use and recharge using potassium chloride rather than standard salt/sodium chloride. The potassium carbonate in the water, instead of calcium/magnesium carbonate also acts as a plant fertiliser as well.

Only issue is potassium chloride to recharge the resin is not cheap, compared to sodium chloride.


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## Hanuman (21 Nov 2019)

ian_m said:


> You can use ion exchange resins to swap out the calcium and magnesium for potassium. Just use a standard ion exchange water softener or you can get ion exchange resin in "pillows" for aquatic use and recharge using potassium chloride rather than standard salt/sodium chloride. The potassium carbonate in the water, instead of calcium/magnesium carbonate also acts as a plant fertiliser as well.
> 
> Only issue is potassium chloride to recharge the resin is not cheap, compared to sodium chloride.


I will see if I can find some anion exchange filter. What are those "pillows" you are talking about?


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## ian_m (21 Nov 2019)

Hanuman said:


> What are those "pillows" you are talking about?


https://www.amazon.co.uk/dp/B002DW0GQQ/ref=cm_sw_em_r_mt_dp_U_W3M1Db5H89EEA


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## Hanuman (21 Nov 2019)

ian_m said:


> https://www.amazon.co.uk/dp/B002DW0GQQ/ref=cm_sw_em_r_mt_dp_U_W3M1Db5H89EEA


Thank you. Looking at the product label it says it only has an effect on GH. How would it affect KH?


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## dw1305 (21 Nov 2019)

Hi all, 





Hanuman said:


> This would be the best bet then. But isn't sodium exchanged in the process?





ian_m said:


> recharge using potassium chloride rather than standard salt/sodium chloride


Like @ian_m says it depends on the resin. The sodium issue is when you use <"strong acid cation exchange"> with Na+ and Cl- as the initial ions.





Hanuman said:


> Looking at the product label it says it only has an effect on GH. How would it affect KH?


Because this is cation exchange, it doesn't reduce the dKH (HCO3- is an anion) but it does reduce dGH (2K+ ions replace one Ca++ ion). Have a look at the Lenntech page on <"ion exchange">, it is a very useful guide.

This is a description of the process of removing the <"carbonate hardness"> using an anion exchange resin, and a slightly <"more in depth one">.

cheers Darrel


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## RisingSun (27 Apr 2021)

I just bought the ion exchange pillow and some potassium chloride.  I want to replace the sodium with potassium chloride.  Can I soak the pillow in the potassium chloride solution immediately, or should I soak the pillow in tap water (with calcium/magnesium) first to release the sodium?


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## RisingSun (28 Apr 2021)

After further research, I found that yes, you can soak an NaCl bound ion exchange pillow with KCl, as the resin prefers Ca/Mg > K > Na

Also, I decided to abandon the idea, as I realized it wouldn't reduce TDS.  Initially hoped replacing the Ca/Mg with K would allow the plants to absorb the K and reduce TDS but the K uptake by plants won't be enough to reduce TDS.


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## dw1305 (28 Apr 2021)

Hi all,


RisingSun said:


> you can soak an NaCl bound ion exchange pillow with KCl, as the resin prefers Ca/Mg > K > Na


That is right. There is more of a discussion in <"Household water softener">.


RisingSun said:


> Initially hoped replacing the Ca/Mg with K would allow the plants to absorb the K and reduce TDS but the K uptake by plants won't be enough to reduce TDS.


You've actually swapped two potassium (K+) ions for each calcium (Ca++) ion, so it won't improve things.

cheers Darrel


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