Beware SERA tests
I tested Kh in the tank and got surprising 13! Just two days before it was 9. Then i figured what's the problem.
I use SERA kh test. The glass has 5ml mark and the instruction says to fill it to 5ml mark. Sometimes i do so, but somtimes i use medical syringe or syringe from JBL test. Now i discovered, the 5ml mark is actually a little less than 4 real ml.
I wrote a letter to SERA about it and got an answer:
Please use the syringe in that case, as these are always precise.
However, the deviations caused by the scale being somewhat dislocated usually do not cause much trouble for aquarium purposes.
Dr. Bodo Schnell
Funny huh? Error of 3 (30% !) dh is no trouble, yeah.
I also have PH and PO4 sera tests. PH seems to work more or less, though i am not sure now.
As for PO4... I added NPK ferts and i shoudl have about 0.7 ppm for PO4, but when i tested it with PO4 sera test i might have got anywhere from 0.05 to 0.25, the color is impossible to identify. Switched to JBL and saw the expected value clearly.
Arghh. Sera, WTF?