Ok here is my input on a closed loop, keep it simple and keep it flexible. A single input and two or three returns is PLENTY! Any more than that and its overly complicated. I would keep the returns low on the back wall, you want to create a gyre(if you don't know look it up ;) ) But basically its a cyclical water flow, and you want the water rotating up the front pane of the glass and back across the surface towards the overflow(GREAT choice on the glassholes BTW I love the one I have on my nano)
Use the vortech to disrupt the flow and keep it randomized.
Just my disclaimer on closed loops, if I were to do my 180 over again I would do it either without a closed loop entirely or a MUCH simpler one. They can be cumbersome, take up a lot of room, use a lot of electricity and are typically louder than than powerheads. Im of the opinion that two vortechs on your sized tank would be better than a closed loop but don't let me make up your mind. Build your tank your way.