Are you using the filter on your KH water intake side for the aquarium? If you remove it does it stabilize? I had a problem where mine got clogged(it was in my sump) and I had to remove it and replace it with a new one.
Second questiion are you doing water changes and if is the water change being done on or around the time of your sample taking?
third make sure your vent tube for both the water water and the one off the back of the unit it not submerge in water.
forth and this was a mistake I did and never throught about it is your ato line close to your KH supply line? This can be amplified also if your using Kalk also. I was going days tearing down the KH and looking, recalibrated the probe each time the pumps and finally caught a break when during one of my KH samples I heard my ato kick on and went please not something this stupid. We did not have graphing of dosing pumps back when this happen but moved my ato line.
Fifth a larger sample of water, I have seen the best results, when the sample is larger then smaller.
I have three running, 1 stand alone and 2 connected to Profilux and after you dial them in they should not cause you any grief. Yes I have had a spike suddenly to 13, do I panic no I just do another test and it goes back to 7.5 and like ok good. I get one of those maybe every 5-6 months. I calibrate my probe every 3-6 months. Except for the one issue I outlined below
Last I was having a problem with one of my units, where it became very erratic to the point o what’s wrong with it. I did a support call and they asked me to take off the cover and noticed the supply lines were clogged. This was because I was not using the filter on the water sample side, I was like it in the return chamber of my sump it should be ok. This took about a year, I wish I took picture cause even the chamber that held the PH probe had gunk in it. I had to replace the tubing and I cleaned the chamber and it was back up and running like nothing was wrong.
PS: it is also very important to keep the distance from the KH solution to the pump to the shortest distance. If your also using a container other then the sticking the dosing line in the bottle make sure there are no leaks in where you attach the tub to the container but also also within the dosing container itself to the cap where you attach your dosing tube. I was helping a friend who was having an issue where the KH solution was not in a bottle he loved being color coordinated with dosing containers and started having an issue and thought his KH was bad.hoe we found it was he started to notice a small air bubble appear, so to confirm he stuck the tube in a new bottle of KH solution and the problem went away.
