After thinking on it further, I should add a legit ato rather than the "me" ato -That’s fine, it’s not so bad to set up.
Once you fill the sump part way with saltwater and turn on the return pump, just start the siphon. I used to suck all the air out with some small tubing.
That’s really it. As you add water, the aquarium level stays constant and the sump water level goes up and down.
Your return pump drives the flow speed.
thank you again!