Fluctuating KH from tap.



The town water I use sometimes changes, it was at 5 deg. now its at 6 deg., I've seen it as high as 9 deg. I was told that it shouldn't go that high again because they had to shut down the oldest well (DER). The only test kit I use is API . I have a drop checker from the Red sea CO2 Pro kit and they say to use my tap water with the solution. As I read the posts here I'm now confused, if I use distilled water and CaCO3 to get a 4 deg. KH wouldn't the reading be incorrect from the drop checker?


Lifetime Charter Member
Lifetime Member
Oct 17, 2006
Phoenix, AZ
With a 4dKh reference solution you would not be measuring your PH or KH, but instead your CO2. The PH and KH are fixed, proven numbers that will not change in the Drop Checker, so the only thing that can change is CO2, which will drive the PH down and therefore change the color. At 6.6 PH in the 4dKH ref solution you know you have 30ppm of CO2, which will show a nice green color. More CO2 and it will go yellow, less and it will go blue.

Your fluctiating KH will mean that your ideal tank PH will change. The drop checker won't care though, as it is only measuring your CO2. Maintain green and all is well.

Tom Barr

Staff member
Jan 23, 2005
And this is a good example why a drop checker is a good thing with varying the KH.

The CO2 flow rate is still the same even if the KH moves due to the needle valve etc.
The plants also have not changed their CO2 demand.

So things should be fine, but careful for the fish, large KH changes are not that good for them. Refill slowly.

Tom Barr