sounds like you have a 90 going into the sump ... can you remove the 90 and replace it with a T ? this will help reduce the back pressure allowing the top of the T to act as an overflow. This is only a temp. solution until you get the time to plumb it better.
If you want to slow down the flow you should use a ball valve on your sump line as well.
__________________
Starting fresh ... 90 gal, 40gal sump, sundial T5HO x 4, 2 x koralia 2's, ASM G1X skimmer 
|