Shawna,
Does your 180 have one corner overflow, or two? This will make a difference. Need more info here.
Regardless, if it is in fact a tank with a corner overflow built in from the factory, it will (more than likely) have one inch drain hole(s). All perfecto and all glass tanks come standard with 1" drains.
A one inch drain hole in the bottom of a tank is capable of draining 800gph. The manufacturers like to cover their butts and tell you it is rated at 700, or possibly even 600 as cully has mentioned.
With that being said, if you have two corner overflow boxes, your tank is capable of draining 1600gph max. This is not taking into account any elbows or head pressure built up in the drain lines going to the sump. Lets be safe and go with what the manufacturers recommend and say you are only capable of draining 1400gph. (Again - this is true if you have two corner overflows, 700gph if you only have one.)
Tailor your return pump to that number, and take into account all the elbows and head pressure losses in the return(s). You say you only have 5 foot of head, but each 90 degree elbow used accounts for 2 feet of head.. and you have to use at least two if you have two returns. Also, if you "T" the mag 18 to two returns then you are doubling whatever head pressure you think you may have. I would like to hear some more info to confirm this. There is a head pressure calculator on the RC main page. Here is a link.
http://reefcentral.com/index.php/head-loss-calculator
If you are already maxed out on return pump and your drains cannot handle any more flow (which is probably not the case) then just add some power heads for more flow in the display.
Hope this helps.
