[GuySmilie]

Your problem is a simple one.

Your pump is pumping more water into the tank than the diameter of the return paths most restrictive element can accommodate. Which in your case seems to be the I.D. of the bulkhead.

Is there a particular reason why you need so much water flow in your system?
If not, then your best bet (most economical) is to reduce the flow coming out of the pump. Do this either by adding a simple ball valve or by restricting the diameter of the tubing connected between the pumps output port and whatever you are using as a termination at the tank end.

Your second choice (more expensive) is to increase the diameter of the water path from (and including) the bulkhead to wherever the water is draining to (assuming it's into a sump below the display tank).

It's really no more complicated than that.