Two must-have links for reef plumbing:
flexpvc.com
savko.com
Both charge an arm and a leg for shipping, so make sure you check that. But both have a good assortment of stuff and decent prices. Also, usplastics.com has some stuff, so it's worth checking there, as well.
The "schedule" determines the thickness of the material, pipe and fittings of the same diameter but from different schedules are totally interchangeable - you can glue sch40 pipe into sch80 fittings. The parts are interchangeable because the outside diameters are the same between schedules - which means, thanks to the thicker walls, the ID's of schedule 80 pipe are smaller, which means a bit less flow. You cannot assume that sch80 fittings are automatically more restrictive, since some manufacturers build thickness on the "outside" vs. making the ID smaller.
In general sch40 is more than adequate. The only time I default to something heavier is bulkheads, since the typical black ABS bulkheads so prevalent in this hobby totally suck IMO.
It's hard to generalize about flow restrictions though, especially with valves, since there are MANY MANY variations. A sch40 valve off the shelf at Home Depot won't, by default, have the same ID down in the valve body as a valve from Lowes - the only thing that's guaranteed to be the same is the size of the fittings at either end of the valve. There are "full flow" valves that typically maintain the same ID as the pipe size they represent, and there are "compact" valves that typically have a smaller ID.
Pipe is a little more straightforward. Sch40 pipe from one vendor is pretty much going to be the same as that from another vendor. Which raises a good point - IMHO, for pipe itself, there's absolutely zero reason to ever use sch80 in a reef tank, and sch40 is probably overkill as well. Especially in smaller diameters. If you shop at a vendor that sells SDR21 PVC, it's worth a try. It has even thinner walls than sch40 - it's still plenty thick enough to be durable and easy to work with, but you get an extra few percent flow without having to upsize all your pluming.
