90 Gallon Build

I am working on getting a 90 Gallon tank set-up going.  The other day I got the tank filled with water and sand then turned the main pump on and everything worked great with the exception of a small leak that has been fixed.  The issue I am having is that I have a galss-holes.com over flow rated at 1500 GPH and I have a 1800 GPH MAG 18.  I thought that I would loose enough flow through my piping which is two 4' runs of 3/4" to the top of the tank; but I seem to be pumping the water out of the sump faster than the over flow can return the water.  I have ordered a gate valve to reduce the flow from the pump.  The question is do I put the valve in line with the outlet of the pump or do I add a T to the outlet and have the valve just recirculate back into the sump?

 

Sounds good, I will give it a try next week when the valve comes.

 

What size put would you recommend? I had read that I should have around 10 Turn overs an hour. I had expected to only get around 900 Gallons per hour out of this pump.