Rd 50 back pressure?
 

How much back pressure can i put on the rd50? Running 1" return line split for 2 returns into tank. I was wondering if i reduce size the bulkhead going into tank to 1/2" cause a strain on pump. Its a 100 gal tank and assuming max flow will only be in the 650 gph range. Wanted to try a random flow generator nozzle to reduce number of intank powerheads (mp40)

Max head is around 130 inches. The half inch nozzle will definitely add friction loss and create additional head pressure as a result. The flow charts are below in this link.
https://royalexclusiv.net/images/con...edy_50_eng.pdf

You can use this head loss calculator to get a ballpark on your total had loss. Select the Royal Exclusiv RD3 50 from the drop down.
http://www.reefcentral.com/index.php...oss-calculator

Would 3/4 outlet be a better choice? According to calc 650-700 you output. Was not sure if nozzle will step on pump

Yes, 3/4 is better than 1/2” in terms of reducing friction losses. I’d expect the nozzle to have a bit of an impact as well but not sure how much.