If you use a large pipe (say 12" to 24") as a fish highway for small fish to transfer from one area to another, you need to drill LOTS of holes (2" or so) along the way in the pipe. I see big pipes (pilings) laying on the bottom somewhat regularly when diving certain locations. The fish, sponges, etc will use the first foot or two into the pipe from each end. The rest of the middle of the pipe will be pretty well barren.
As an example, say you have a 16" plastic pipe 20' long running from one brush pile to another. Seems like the perfect fish highway for small fish to transition from one place to another. A solid pipe will hardly get used.
Drill about a hundred 2" holes in it and it will work well I think.
The water inside the solid pipe will get stale. It will be dark in there, even more so if you cram brush inside. Put lots of holes and suddenly 1-2" fish can dart in and out the holes for protection. The holes allow water exchange so there is O2. The holes allow light penetration so algae can grow so more food and O2.
A good tool to make 1" holes in plastic pipe is a multi step drill. Harbor Freight sells 1" and bigger ones. A wood bit works decent but tends to break through and not cut a full size hole. Hang on to the drill. I have found step drills to work much better.
If you want to put some stuff inside that would be fine but I would not pack it tight. Just a few branches or something. Place it above any thermocline or at a level small fish will use.
That is the way I see it in my non expert mind.
step drill These may not be a very good bit for a lot of metal drilling. But for plastic I have had good luck with them.
For bigger holes or "slots", drill two holes close together then connect with jig saw or sawzall.
