Help, can't figure this out

[Coleus]

ok here it is

Here is the screenshot of the second tank



HEre is the overflow box of this tank



Here is the back of the tank



Water is draining into the sump



The return line from the sump

ok here it is

Here is the screenshot of the second tank



HEre is the overflow box of this tank



Here is the back of the tank



Water is draining into the sump



The return line from the sump



I have put some thoughts into what Tony said in the thread, my drain does bend 90 degree and it has to travel horizontal from the ground into the sump. Thus it possible that the air got trapped inside the flexible pipe, eventually it build up enough that stop water from flowing back to the sump thus causing water overflow in my tank. After a 20 minutes or so (my assumption) it has enough pressure to push the air through and back to normal level but by then i got a wet floor already. Do you think my hypothesis is right?

I have reduce the flow of my return line but if my hypothesis is right then it is a ticking bomb