Ok, drained the tank completely and flushed out the filter. I just put some carbon in. After about 8 hr's running the filter had about 1% the bubbles as last night. The amount of air seems to be directly related to the overflow level. If the level drops tons of air starts to show in the filter inlet. Raise the overflow level and air stops.
Removing the drip tray to agitate the water in the filter does not produce more bubbles, so I'm leaning heavily towards too much air being sucked in through the overflow vents.
Now the problem is metering the overflows. I have two rated at 700pgh and a 2400gph pump. But because of the 5ft head and amount of elbows I had to install the pump can't keep up (just barely). I've been trying to tweak the ball valves I added to each overflow. But their not exactly precission controls. The level in the tank creeps in one direction or the other, taking up to five minutes to see if a small adjustment worked.
Getting some feeders before my fishes go in this thing.
EDIT: Now that I've been playing with the levels in the overflows and limiting the amount of air, the bubbles that developed over the last eight hours are diminisioning.