The key is design of the height of the lowest intake or return. If he has a 20 gallon sump, you make sure you can only drain out less than 20 gallons of the main tank
Yeah that's what I mean. If I put checks on the returns then they are out of the equation in most normal occasions and wouldn't be much lower if at all anyway, which normal factor of safety of like 2-3 would take care of. And since tank is designed to regularly drain to lowest intake, we are good there.