pH drop when pumps stop?

[W1ngz]

Just moved my reef into a shiny new 90 cube and sump. I don’t recall ever noticing this behaviour before. When I turn off the pumps to feed, my pH reading drops pretty well immediately. You can clearly see my feeding time this morning and just now in the picture.

Anyone have any ideas? Is this a probe thing, and apex thing or a pH thing?

 

[Anthony Kochevar]

Lower oxygen in the water from lack of movement could do this.

 

[W1ngz]

Lower oxygen in the water from lack of movement could do this.

With 20 seconds? In a brand new sump (online for 5 days) with pretty much no bacteria colony?

 

[W1ngz]

I'm wondering if it's not a voltage drop somewhere causing the probe reading to change.
Gotta walk the dog first, then I'll try turning off the valve on the return without shutting off the pump and see what happens.

 

[W1ngz]

Tested this by cutting flow at the valve instead of turning off the return pump, same result, pH drops by about .2 almost immediately.

 

[W1ngz]

Turns out this is the probe, the reading is much lower when the tip is more than 2" below the water compared to 1". Stopping the return increased the sump level 1", which seems to cause my probe to read too low.
Time for a new probe?