I bought the kind with 40 ft2/ft3 on the advice of the guy that sells them. I think it depends on how clean your water will be (solids and crap). He told me that if I wasn't filtering my water (which I'm not) then I better stick with a pretty open structure. I also am trying to keep it simple and had an old hot tub pump so I elevated my 12 ft3 of stuff above the fish tank and bought also one of the big square pattern spray nozzles from the guy at the web site and am circulating about 40 gpm through it all the time. I think for the number of fish I have (only 100 in 1800 gallons) that this will hopefully give me enough oxygen and biofiltration area to keep up with the ammonia.

The verdict is still out on what I'm trying. I don't think the bacteria is fully established yet because my water is starting to smell sort of fishy. I am doing some major water changes right now to try to clear it up. The water goes on the stuff in the greenhouse. I also am not circulating my water through any gravel beds yet. If I was that would probably help. People on here are basically saying that the thing to do is circulate teh water through a grow bed and that will become the biofilter - no need for the structured packing. I just didn't set up that way - yet.

I do think that bubbling underneath the structured packing would probably do a better job of aeration than what I am doing (I would bet my paycheck on it) but I didn't have an aerator and I did have a pump.

I think what you are saying is right - just get a good sized aerator and some big air stones, submerge the blocks and then bubble air underneath. The air will lower the density of the combined water+air in the blocks and that will cause large amounts of water to be pulled up through the packing (blocks).