Well, I am using a standard reef ready, 65-gallon tank. The permanent overflow does have a 1-inch drain (to the sump) and a 3/4-inch return.
I am using a MAG 7 pump. That pump has a 1/2-inch discharge size, which I had to purchase adapters to fit it to the 3/4-inch ID flexible hose I used for the return line.
The only place I guess that air could be getting in is in the fittings connecting the outer part of the sump bulkhead fitting to the return hose. There is a very slow (about 1/4 cup over a six-hour period) water leak there, maybe one drop every 5 seconds. However, due to the sheer volume of microbubbles in the DT, I don't think that can be the sole source. The return hose is see-through, and I can see no air bubbles in there. The pump compartment now maintains about 4 inches of water too, and that water looks quite bubble-free to me. No air is getting past my bubble trap. I will try to post pictures/drawings of the system when I get back home.
My skimmer is now turned off, mainly because it has not cycled yet, and was constantly filling up with clear water instead of skimmate. I am told that it just takes a break-in period for that to right itself.
I think the bio-load for a 65-gallon tank is very moderate, with the following:
Sailfin Tang x 1
Blue damsel x 3
Green Chromis x 1
Diamond Goby x 1
Six-Line Wrasse x 1
Horseshoe Crab x 1
Nassarius/Cerith/Astraeus snails, probably 45 in all, 5 of the Astraeus.
I need to drill that hole in the return pipe - where do I need to put that in relation to my current overflow water level? I have the Durso pushed all the way down as low as it can go, which keeps the overflow slightly higher than half full.
Where the water enters the sump, there is a constant layer of bubbles there too, although they are not going past the bubble trap just upstream of the pump compartment.
Right now I am just keeping an eye on a little bowl placed to catch the dripping from that leak mentioned before. I secretly fear that I may have to remove the sump to fix that, which means diverting a LOT of water to buckets or water cooler bottles. I do not want to do that until I figure out my microbubble problem though.
Thanks for all who are focused on my issue.
Shane