New sump

[Vagabond]

My tank is RR so the overflow is built into one of the corners and runs
6"x 6" with a 1 1/2 " drain. I return with a Mag 12 on a 3/4" return line that is T'd to two 3/4" returns into the tank. I have never had a problem with the tank over filling, in fact, I could probably go to a Mag 18 if I wanted to increase flow through.

I think Brad has a valid point in the teeth perhaps being too narrow and restricting your flow. There is also a correlation between the strength of your return pump, drain size, and the linear inches of overflow. I have 12" of overflow so it can handle a fair amount of return. Your drain size is more than adequate IMO at 2" so I suspect that yes, it is a shortage linear overflow. You may try a valve as Brad suggested or possibly filing in between the teeth to increase the size of the gap allowing more water through.

HTH