[mark]

I'm with others, might be to do with volume.

To set level; with power on, fill display from sump until starts to overflow into sump, power off, let back flow until display stops draining, fill sump to the maximum level your comfortable with, mark max. Turn on pump, opening valve until until your drain flow max's out, mark water level in you sump say as max, power on. Your pump should be able to run without going dry to the 'max, power on' level, if not, your sump is too small. You can run your system below the 'max, power on' level to set heights for skimmer depth etc, not above.

With evaporation and not topping up (or having a ATO) you'll eventually draw air. With power on, never top up above the 'max, power on' mark.