Powerhead recomendation for River-tank manifold

Choosing 1/2" limits your flow considerably. I'd recommend MJ1200s. They'll move almost 300 gph ea, minus losses from all the elbows in your manifold. Powerheads with intakes larger than 1/2" won't flow nearly what they're rated at because of the restriction of the manifold. I'd also say that if you scrap that manifold and use those same MJ1200s set up for circulation you and your current loving fish will be much happier. You can get a circular flow pattern pretty easily with two units, and you'll have over 3500 gph total flow. I did a standard 30 with a SEIO 2600 for Rainbow Shiners and Rainbow Darters a few years back, and 2600gph in a 30 wasn't too much flow. Not sure what type of Hillstreams you'll be keeping, but they're all at least as highly modified as Darters to handle high flow. High flow rates bring out interesting behaviours in fish. My Shiners always schooled very tightly, and would hang out and seem to play in the direct current, then suddenly on cue the whole group would duck into an area of lower flow where they'd hang out a while before going back out into the flow. And watching fish chasing food under such conditions is a blast. The PVC manifolds are helpful in providing a linear quality to the flow, but limit quantity greatly. I've thought about a design where there's a raised bottom and two side walls inside the tank, all sealed. Essentially a tank within the tank. Behind one wall place modded MJs, SEIOs what ever, through holes cut which allow the output end to protrude, but allow the water inlet slots to remain on the back side of the wall. The wall on the opposite end has screened openings to allow the water to flow in, down, across the bottom and back to the pumps. Linear flow and lots of it.

I would go with at least 3/4" and better 1". Then you could run much more powerful units than the MJs and get the full good out of them. 600 gph in a 4' long tank really isn't that much movement. If you stepped up to 1" and ran something like a pair of Mag 9s you'd be talking some flow!

600 gph = 10 gpm. that's seems like a lot of water to fit thru

this site:

http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml

predicts that 3 X 1/2" would work with low pressure, but remember that all you have pushing the water is a few inches of head. i'm with subrosa, kick it up to 3/4"; power heads (and almost all pumps and fans in general) really don't like having restrictions on their intakes.

alternately, could you lay a sheet of glass/plexiglas over the top of that structure, slot all the end pipes, and create a wide plenum for the flow?