Kh testing


Jan 24, 2005
Melbourne, Australia
I know this question has possibly been asked before but what other things affect Kh test kit measurements of aquarium water. I ask this because I will be demonstrating the Co2 drop checker / Kh reference method in one of our members tanks at our plant group meeting this friday and someone is bound to ask the question. :) 'Why dont we belive the Kh reading when testing aquarium water.' :confused:

Tom Barr

Staff member
Jan 23, 2005
Hi Paul,

Ask the reverse:

Why should we assume that it measures just KH?
Do we really know the answer?
Is that really a safe assumption or not?

How can we safely say and assume that there is no other influence given that according to the pH/KH chart, some aquarists supposedly have 100-200ppm of CO2 and healthy fish and plants?

While others have issues at 40 ppm?

In both cases, teh pH probe was calibrated. So that 1/2 of the issue has been resolved. But even there, acids can influence the pH and not influence KH.

There are other things than bicarbonate that influence alkalinity. Test kits measure total alkalinity, not just bicarbonate. Seachem makes a few test kits for Borate and other alkalinity for marine systems, but few are available for hydroxide alkalinity etc.

The way to measure the non carbonate fraction in a sample is indirect:
You make a reference sample using sodium Carbonate only + DI water.

You compare the reading ppms to that of the unknown sample.
Did it take more acid to tritrate the sample than the reference sample?

If so, then the remainder is non carbonate alkalinity.

Note: adding more non carbonate alkalinity gives you less CO2 ppms than the pH/KH table would have predicted.

Also, adding peat or other weak acids also gives you less CO2 ppms than you would have predicted as well.

So in both cases= poor CO2= algae and other plant issues.

Some folks also reduce their surface movement down way too much, more is better, then they can simply add a bit more CO2 to make up for the loss. this adds good current and mixing as well as reduce addign too much CO2 and adds importantly O2.

Do not rely solely on plants alone for O2.
Bad idea.

CO2 is easy to add more.

Many add less CO2 or cannot measure it well and then think it's their nutrients giving them issues and spend sometimes years fiddling with that.

So this is a classic example/s why correlation does not = cause.

Tom Barr