Most algae reactors are for low pressure, slow univeral flow.
For Biopellets you want all the water forced to the bottom to fluidize the pellets. It's usually under higher pressure because you're pushing more flow thru it to keep things tumbling.
Now I'm talking about a big reactor for a 250g+ tank. If this is for a small tank and you're talking about a lil 4"x12" reactor then it probably doesn't make a difference as long as you can force the water to the very bottom.
Dave B