Hi,
With regards to your statement "Maybe beneficial to use black pvc to keep watever light out"
No light will make it thru any Pipe. Also keep the pipe Ø as big as you can for as long as you can for the simple fact that over time calcification buildup will restrict the flow and it's always easier to have a reducer at the either end than to have the whole length of pipe clogged up. And a closed loop (this is just my opinion having run both at the same time including using a Manifold return) is not as random as power heads nor as Economical when you consider the long term cost of your electricity bill. But if your gonna do a closed loop with a powerful pump consider running pipe inside the tank down to near the bottom or putting flow there to keep the detritus in suspension to be better removed by your sump sock/protein skimmer. Also You don't need allot of rock as history has said we do scientists have measured the bacteria levels and determined that after a cycle is started there ALLOT in the plumbing pipes themselves and more is not necessarily better, However it looks better to most people to have a large wall instead of a few Columns and a Thin long shelf all well adhered together to provide shelter and different light intensity's.
Paul L