Overflow Question

I have a 90 gal. glass tank with corner overflow. The overflow is 3" X 6"
made of glass plates with a plastic weir on top. There are 2 - 1 1/2" holes
in the bottom of the tank leading to the sump.

My problem is that with my Little Giant pump, supplying return through 4
holes located around the tank bottom, the flow of water over the weir has
lead to the death of several fish.

I want as much flow as the pump can supply but don't know how to keep the
fish from going over Niagra Falls.

BTW, the water going over the weir is no more than 3/8" from top of tank.
If I valve down the flow to the pump, will that hurt the pump?