# Low KH



## BigDaddy (31 Mar 2014)

Its 5 °dKH _(89.5 ppm)_ coming out of the tap yet only 1 °dKH_ (17.9ppm)_ in the tank 2 days after a water change

Why is it dropping so low?


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## jose_j (31 Mar 2014)

Hi BigDaddy, it is very likely that your substrate is lowering the KH


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## BigDaddy (31 Mar 2014)

It's just bog standard aquarium gravel thats been in there for yonks and gets a good hoover when doing a water change


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## Arne (31 Mar 2014)

What's in your filter ?


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## Spnl (31 Mar 2014)

It's hard to imagine how gravel could lower KH, increase it perhaps.
So, probably your KH is not changing. Did you measure it using a standard test kit? The kits that titrate the KH actually titrate the alkalinity in the sample. 
So your 5KH could be an artefactually high reading, caused by eg NaOH or phosphate in the water, and that is neutralised in the tank. KH of 1 being nearer the truth.
That would be my guess anyway.


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## dw1305 (1 Apr 2014)

Hi all,





Spnl said:


> So your 5KH could be an artefactually high reading, caused by eg NaOH or phosphate in the water, and that is neutralised in the tank. KH of 1 being nearer the truth.
> That would be my guess anyway


 My guess as well, and probably NaOH, they add it to nearly all soft water in the UK to raise the pH (they also add PO4---, but that isn't such a strong base).

cheers Darrel


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