KH Suddenly Decreasing at night - possible reasons.

[Dibi]

The device you are using MUST fully aerate the test samples to get alk by pH. If that process is incomplete for any reason, then it will report higher alk at higher pH, even if alk is steady. The higher the pH peak, the more it will be off at that peak.

Dosing carbonate instead of bicarbonate raises the pH (as you found), so if your device does not fully aerate the samples it is testing, the switch to carbonate will give false high alk readings when the pH is highest.

Remember, the alk effect day to night you are seeing is fairly small.

Perfect I got it! The device try to aerate, it takes like 30-40 minutes to do the test. But it started to grow some algae inside the chamber due to lighting, I will clean and do a factory reset and see if something change. If not, maybe this device does not work wel with high PH.

 

[Dibi]

Perfect I got it! The device try to aerate, it takes like 30-40 minutes to do the test. But it started to grow some algae inside the chamber due to lighting, I will clean and do a factory reset and see if something change. If not, maybe this device does not work wel with high PH.

Or simply by dosing carbonates instead of bicarbonates my reef is actually consuming alk at night.

 

[Randy Holmes-Farley Verified Community Expert]

I would also add that this method is also

Or simply by dosing carbonates instead of bicarbonates my reef is actually consuming alk at night.

That seems unlikely if it previously consumed during the day. You did not change the total amount of bicarbonate vs carbonate in the water much by making this change and raising the pH a little.

 

[Timfish]

I didn't. Do you think is the reason?

Is really the only change recent.

Switching back will give you more data to help figure out if it's a testing failure or something else. Dissolution is happening in all marine systems all the time through biotic (biological) and abiotic (nonbiological) processes (up to 1 kg CaCO3 eroded m−2 year−1) I would suspect what your change in dosing might have shown is some of the dissolution that is occuring all the time, dosing bicarbonate was just masking it. Two jpgs of the dissolution I've seen in my systems:

Dissolution of Birdsnest skeleton after tissue recedes from lack of light.

These Murex shells from Thin Stripe Hermits I get from the coast and are in near pristine condition initially.