the 2 J tubes should have drained a total of about 1600 gph. the J tubes go into your tank and inside of a small box with teeth at the top.
was the water rising above the top of the teeth?
did you try lowering that box with teeth deeper into your tank?

maybe the J tubes drained the water from that box faster than it could be filled, so the bottle neck is not the J tubes but the water flow going into the box with teeth. good luck.