If readings are fluctuating more than you expect, work through this chapter before recalibrating. The causes fall into two groups: mechanical, tied to the equipment and the sample, and chemical, tied to your aquarium water. In the vast majority of support cases the cause is in the first group.
13.1 Mechanical causes
Cause
How to identify and resolve it
Unequal aeration between the chambers
The most frequent one. Check the side valves: both chambers need strong, equivalent airflow, with at least 5 mm of foam. If the air pump is weak, clean the air system or replace the pump.
Check Probe Health (Chapter 12). Below 55%, or more than 12 months in use, replace it.
Stray voltage in the water
Current leaking from some electrical device in the aquarium interferes with the probe reading. Test by switching devices off one at a time.
Unrepresentative sample
Move the hose tip: away from detritus, from the skimmer and from the balling dosing point.
Sharp temperature swings
Move the equipment away from the chiller or any heat source.
13.2 Your aquarium chemistry
This group of causes is less common, but it is usually the most confusing when it shows up — because the equipment is still working perfectly and yet disagrees with your chemical test. It is worth understanding why.
The table below gathers what is worth investigating when the test and the equipment disagree, with the normal range for a reef aquarium and the order of magnitude of the deviation — in dKH, so you can compare it with the difference you are seeing:
Parameter
Normal range
Investigate from
How much it weighs
Boron / borate
4.0 to 5.0 mg/L
above 6 mg/L
It is the second largest buffer in marine water, behind only carbonates, and the most common deviation in practice. In your test, about +0.09 dKH at 6 mg/L and +0.34 dKH at 10 mg/L — without your carbonate reserve having changed
Salinity
34 to 36 ppt (1.025–1.027)
outside 33–37 ppt
About 0.06 dKH for each ppt of deviation. The chemistry of the whole of marine water depends on salinity, so it is worth checking the refractometer before suspecting anything else
Dissolved organic matter
clear water, active skimmer
yellowing water, skimmer stopped or undersized
Chemically, little — on the order of 0.1 dKH. The real weight is mechanical: organic matter fouls the chambers and the probe, and then the problem stops being chemical (section 13.1)
Temperature
24 to 27 °C, stable
swings above 2 °C during the cycle
Chemically it is small, around 0.01 dKH per °C. What gets in the way is the variation during the cycle, which keeps the reading from settling — hence the recommendation to keep the equipment away from the chiller
Worn pH probe
Probe Health above 75%
below 55%, or more than 12 months in use
A drift of just 0.01 in the pH reading is worth about 0.2 dKH. It is the single cause that shifts the result the most — and the easiest to fix (Chapter 12)