OK, refer back to the pic posted earlier in this thread.
See the vent hole shown? this is for a siphon break. It lets air in and when the level of the tank drops below this, it lets more air in, causing the siphon i the downpipe to be lost BUT it keeps the water in the overflow pipe over the side of the tank, so when tank level rises above this vent hole level, the drain will automatically start
Without it, the overflow will completely drain the tank to the intake level, where it sucks in air, just like your Youtube vid does. Start with a 1/8" diameter hole and see how it works, if still same, try 3/16" What some people do is put a piece of airline in there and a John Guest valve, and can vary the valve depending on drain requirements. But fiddlin with drillbit holes will work good too