"Jerry Baker" wrote in message news:upcRg.706$Vk4.257@trnddc01...
I have a new 125-gallon glass tank with a built-in overflow. The problem
is that, even though the tank is level, the water level stays below the
plastic rim of the tank. Is there anything short of modifying the
built-in overflow that I can do to increase the water level in the tank?

I tried adjusting the level of the water inside the overflow and that
has no effect, and adding more water just raises the water level in the
sump.

What pump do you use? Does it have enough flow ?
Usually, the stronger pump = higher water level in the tank.

Try to find out for how much wate flow is your overflow rated
and pick a pump which delivers flow near this maximum at
a given hight. (flow decreases with the pumping hight)