Hi,

Ideally the probes should be placed in an area where there are no microbubbles present. If that is not really option, an area with minimal microbubbles is the next best thing. You don't want these bubbles sticking to the probe tip because that is when you may see odd readings. The refugium is fine as long as you don't allow algae to grow too heavily on the probe eye.

If your return section water level is stabile, you can put the probes there.
Just keep in-mind if you are using the P4 for AWC, you have the option to prevent the sensors from triggering an alarm for the short time they will be out of the water during a water change event. That option can be found in the settings page for each probe, No alarm on water change.