8O 8O You've got some serious water turnover there with 1800 gallons (maybe 1700 with head loss) of water running thru that pump every hour. That means with a 15g sump - (assuming a 20g with only 15g of water in it), you're emptying it about 100x an hour. The main tank is being emptied/turned over maybe 35x an hour. I see why powerheads are of no use to you and provide too much current.
How's your filtration set up? Skimmer and how situated? What I'm wondering is if the water is moving too fast for filtration between the sump and main.
I think the "circulation" is purely water movement within the tank, like with powerheads. "Turnover" is water movement between the sump or wet dry. I think I got it right. But circulation is commonly faster than the turnover. You've got all your in your turnover. Not sure that's working best for you, but I'm sure there'll be opinions coming soon. I kinda think 1800 gph
could be ramped down to a Mag 12 or maybe even less, and then place a couple of random or sweeping powerheads (like maxijet 900-1200's) on either end of the tank. Maybe your reef won't be as over swept as I think you've described.