I know this probably isn't the direction you want to take the thread, but why are you so convinced you need vinyl and barbs? There are a whole lot of specialty fittings out there for tight quarters. Look at savko or flexpvc.com for inspiration. You might yet be able to hard plumb.
Overflow box, centered on the tank, has three down pipes, including one that comes right down the middle; the closed loop exit is also located smack dab in the lengthwise middle of the tank. Hence, the plumbing interference. (I had sent the overflow to Coast to Coast when they were fabricating the tank, so they could get the holes for the overflow box positioned correctly; you would think they would have spotted this........ yet another reason I think they suck.......)
Even "swinging the downpipes" to the side using a pair of 45 degree fittings (jog over, jog back down), there is interference. I shaved down a portion of the socket collar for the lower 45, and it just clears the shank of a barbed elbow screwed into the closed-loop outlet bulkhead. A socket-type fitting, on the other hand, which would be used to hard-plumb a pipe to the outlet, is just too thick and blocks the jogged-over downpipe. (I can't use a pair of 90's to turn full lateral then back down, since the downpipes are two close together.) Hence my need to go with vinyl tubing.
Re flex pvc, I am aware of it and have been planning to use some of it to make it easier to line up the plumbing to go through holes I will be putting into wall behind the tank (fish room on opposite side of the wall). And that could help me bypass the closed-loop outlet. Two concerns, though. One, I worry about torquing stress that would put on the bottom of the overflow box, via the bulkhead being torqued, and two, all the fittings I"ve purchased have been schedule 80 for the gray color (the plumbing behind the tank will be visible from teh side, before it enters the fishroom behind the wall), whereas flex pvc all seems to be schedule 40 so won't seal well in the sockets of the fittings I've got.
