Well...it depends on what size pump you intend to use. I would recommend using whatever size pipe that the pump outlet is. For example, a Mag 12 has a 3/4" outlet so I would use 3/4" pipe on it. I'd also use PVC
instead of hose. You can put a couple of unions in there so you can get it apart to clean it and a valve for pump removal. It makes for a neater installation and less chance of a leak. If you can get the thinwall PVC
(Schedule 20), that's better for aquarium uses. The best setup would probably be a drilled sump with an external pump like a Iwaki or a Little Giant. I think, for a 75, something in the 8-900 gph
range would be good. Champion Lighting has a new line of pumps called Blue Line that are supposed to be good.
You will have to have light to grow the macro algae...it'll die without it.