When I start my 90g with overflow box, do I fill my sump, turn my return pump on then fix the level from there? Very confused and don't want to flood my living room
My suggestion...fill the sump to the expected "normal" operating level. Add a little extra if the overflow box is empty. Turn pump on and be ready to add additional water to bring the sump level back to where you want it.
Keep in mind to leave room in the sump for back flow when the pump shuts down. You can adjust the amount of water that back flows by adjusting the depth of your return lines in the display. The nearer they are to the surface, the quicker you will get a siphon break, thus less water back flowing.