Sorry for the tardy reply. Been busy and missed reading some posts for a few days. The pipe is mostly 2" because that is what scrap I had laying around. Had one piece of 3" in there too.
The holes were drilled with a step drill and drilled to a size I thought was "about right". They are all different sizes. I doubt if they are "important" but my line of thinking was to allow more water flow for better O2 levels and to give the FHM escape routes from predators. It is probably more about my imagination than anything really useful.
I did try to point the pipes in directions so the competing males would have some privacy and be less likely spending their time fighting for turf from other males. The 2x4 up the middle blocks the view of tubes on the other side row.
In reality I suspect the FHM will use the rock piles more than the condo but I likely will never know for sure. I have spotted one male protecting a nest in one of the rock piles.
They are successfully spawning because I have noted lots of 1/4 to 1/2" FHM fry in small pools around the edge of the pond.
