From the Aquaero you mean?
The header you want is called Tacho in the Aqua Computer diagrams and it's on the right bottom if you are looking at the rear of the Aquaero. It takes a normal 3 pin female fan connector.
There is also an rpm headeer on the Power Adjust units. Probably on other Aqua computer parts as well but you get the idea, just look at the diagrams for the header layouts.

It will show up as a fixed rpm if plugged into one of your motherboard fan headers. Some motherboards can shut down on loss of rpm signal on cpu fan header, others just give an alarm.

RPM signal is independent of fan channels, and does not increase with any fan rpm.



Need to set up alarm configuration and then set alarm action in Aquasuite

But as you have not indicated what hardware you have

here is what I do with my Aquaero 5 Pro:


On alarm configuration page:
Fan channel 1( for me master pump) if no signal for 6 seconds "Warning", if no signal for 12 seconds "Alarm/warning 7"
On Alarm action page:
Alarm /warning 7 turns off tacho signal and also loads profile 4

Profile 4 is very much like profile 1, but only red LED of the Tri-LED is functional and is flashing. The secondary pump's alarm/warning 7 in profile 4 is tacho off and relay switched ( equivalent to press case start power button) to force shut down.