you have an overflow box, right? don't restrict the flow going from your overflow to your sump. your overflow box ( 1" bulkhead ) will determine the max amount of flow your tank can handle. that should be around 600 gph without any elbows. all you have to do is adjust the return flow so that it is at, or slightly less than, your drain. you may want to take some water out of your sump so that: 1) if your drain clogs, there isnt enough water for the return pump to overflow your DT or 2) if your return pump should lose power, the return line wont create a siphon and flood your sump