[Enigma]

Maybe remove the surface skimmer. Something must be impeding the water flow from the display into the back chambers, or the water flow through the first back chambers into the return chamber. That is the only reason I can think of why the display area of the tank would be overflowing.

If you had upgraded the pump, my guess would be that the pump was pumping water into the display area more quickly then it could drain into the back chambers. If you're using the same pump you have been all along, I don't think that should be the issue. If anything, I'd suspect that the volume of water the pump was moving would decrease over time: not increase.

Another thought . . . I have an NC12 and an NC24. There is a little hole in the divider between the display and back chambers. It is beneath the the intake/overflow grill. Did you silicone that hole closed? That's a popular mod, and one I was actually going to do. I don't know if closing off that hole would make a huge difference to the flow or not.