Rebuilding hardware set up

[pterfloth]

That could be the solution. The lines in and out of the Fluval don't matter since it is enclosed. Connect the front tube to your return. It actually looks upside down. On mine, the "U" sits over the tank lip with the flared end just below the water surface.

I suggest turning off your return pump and quickly disconnecting the return hose from long line at the back. Loosen any hose clamps first so you can do it quickly. Once the line is disconnected your chance of massive draining should eliminated. Turn off the Fluval and swap the lines. Try inverting the "U" so it sits over the tank lip.

I think this should work. The overflow will still drain some so you do need some room in your sump for that.

Let me know how it goes. Good luck!