a closed loop is just an apparatus that takes tank water and pushes it back in with force and direction. the best way would be to drill a hole in the back of the tank for an intake. attach the pump to that, and then plumb a few returns off of it, to the tank. another way would be to use an over the top style closed loop. this would have to be primed though. there aren't many self priming pumps available to the hobby that would be good for this situation.
here are some pics to give you an idea. this was the set up of my 195 in wall. at the bottom you see the reeflo dart (later changed to a barracuda) and that pushed water via a 1.5" pipe, to the above "oceans motions 4 way", and then to four 1" pipes that headed back to 4 spots in the tank. i had the tank builder drill a ton of holes in the back of this tank. if i had to do it over again, i would have made a lot less holes and came over the top with the returns. anyway, in this situation, the pump ran constantly, and the OM 4 way diverted water to different parts of the tank at different times, making a random turbulence. it's not necessary, but it worked awesome.
in hindsight, not only would i have drilled less holes, i would also have used spa flex instead of hard pvc
, to cut down on friction and head loss, and put more flow in the tank.