If the return pump was sucking air and the display tank was about to overflow then the combined flow rate of both your overflows is not capable of handling that mag24 and it is emptying the sump into the display. OR one of your overflows lost syphon. Were both of the overflows still functioning when the incident happened?
overflows can work well enough but they are sketchy. A 1.5" schedule 40 PVC
pipe "should" be able to do 2100gph at full syphon under gravity. But that does not take into account all the twists and turns of the overflow design. IMO
, do yourself a huge favor and get a proper HOB
overflow. It could very well keep a huge mess from occurring and will give you a measure of peace of mind. If you stick with the dual DIY overflows you are going to need a couple few days in close proximity of the tank so you can tweak the return just right.
Hopefully someone who has a dual DIY PVC
overflow rig will show up to give you some more tweaking tips.