Steve, this is definitely one of the best threads I've ever read here on PT. I'm just now realizing that I've spent most of the night poring over the details of your amazing setup. You've done an incredible job with every aspect of this tank. Just looking at your photos of the fish and the aquascape shows how well it's paying off. Fantastic job.
I'm sorry but I had to laugh when you mentioned the issues you had with your OC canister leaking. I just set up a dual OC system (simple series plumbing) for a client and had the exact same issue with both canisters. This was my first time using OCs so, at my wit's end after trying to resolve the leaking problems with no luck, I called Ocean Clear hoping that they would have some advanced technical advice on how to fix things. I spoke to a woman named Pinkie who, after I gave her an exhaustive description of my problem and the lengths I had gone to to resolve it, told me that I just needed to whack them with a hammer to seal them. Problem solved. Unbelievably simple
I've had GDA problems similar to yours in tanks I've set up that use MH lighting (both FW and SW), however I rarely have serious GDA issues in tanks with fluorescent lighting. Although my evidence is only anecdotal, I suspect that the more intense, focused nature of MH lamps contributes to the likelihood of GDA because it causes much more glare from the inner surfaces of the tank, making them a ripe target for algae. I've got a 125G planted tank with 2x250w MH lamps over it that requires regular cleaning to prevent a film of GDA from covering the glass. I also find this to be the case in many MH-driven reef tanks.
I've recently had good luck in reducing the build-up of GDA in my planted tank by cutting back on trace/Fe dosing. My current theory is that excess micronutrients (iron is my primary suspect because I like to dose a lot of it), in the presence of very intense light, can lead to GDA problems. In the last two weeks I've cut back my trace/Fe dosing by 50% and have seen a considerable decrease in the amount of GDA on my glass. Again, I have no hard evidence that this is the case, but from my experience it's a strong possibility. It's possible that by adding all of those stem plants you did something similar, in that the fast-growing stem plants you added helped pull excess micros from the water and deprived the GDA of one of its primary drivers. And the additional filtration you've recently added definitely doesn't hurt.
It's also possible that Maracyn has had some effect on your GDA (and if that's the case I'm going to buy a bucket of the stuff), but I have my doubts. Maracyn is basically just erythromycin, an anti-biotic effective against some types of gram-negative bacteria. BGA (not actually an algae at all but a cyanobacteria) happens to be one of those gram-negative bacteria that is unusually susceptible to it. I would be surprised if erythromycin is as effective against true algae like GDA, although I could be completely wrong on that one.
In any case it's good to hear that things are looking up. I'll be following this thread closely from now on. But right now you've inspired me - I'm off to shop for Mazzei venturis