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KH Stability

John P Coates

Member
Joined
21 Feb 2014
Messages
164
Location
Bracknell
Hi Folks,

In my tropical planted tank (including fish), I use RO water to which I add Tropic Marin Re-Mineral Tropic salts. I also add sodium bicarbonate to bring the KH and GH to 5°dH and 7°dH, respectively. In as little as 9 days, the KH has increased to 6.5°dH. When KH = 6.5°dH, pH varies from 7.7 to 7.9 as CO2 is injected throughout the day. I would like to get pH down to 7.0 or slightly less but the higher KH is making this difficult.

As an experiment, I made up a litre of water using RO to which I'd added sodium bicarbonate only. I made this up to achieve KH = 2°dH. To my surprise, I also found that, after a week or so, the KH had risen to 3°dH. During that time, the water has been stored in a plastic bottle (with lid) at room temperature in a dark place. KH was measured using NT Labs and Nutrafin test kits.

What is causing KH to increase over time? Isn't KH supposed to be stable?

JPC
 
Last edited:

John P Coates

Member
Thread starter
Joined
21 Feb 2014
Messages
164
Location
Bracknell
Fellow aquarists,

I was hoping to have got some response to my question. As there are many factors that could affect KH in an aquarium, that is why I set up the control as explained in the second paragraph above. Despite this simple experiment, I still found that KH was not stable. I have made up another KH = 2°dH sample using 60mg +/-5% sodium bicarbonate per litre RO water. I use this 2°dH sample in my drop checker as I aim for 15ppm CO2. I will re-test the sample at the weekend to see if the KH has changed.
 

dw1305

Expert
UKAPS Team
Joined
7 Apr 2008
Messages
12,350
Location
nr Bath
Hi all,
As an experiment, I made up a litre of water using RO to which I'd added sodium bicarbonate only. I made this up to achieve KH = 2°dH. To my surprise, I also found that, after a week or so, the KH had risen to 3°dH. During that time, the water has been stored in a plastic bottle (with lid) at room temperature in a dark place. KH was measured using NT Labs and Nutrafin test kits.
I wouldn't worry about it, but it is the test kit, or possibly a residue in the bottle, as you suggest sodium bicarbonate (NaHCO3) is pretty soluble, so there isn't any reservoir of alkalinity that could cause the dKH to rise.

cheers Darrel
 
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