Very nice Geoff. A couple things....(besides the discus, I'm sure someone else will touch on that.) With DIY CO2 in a 55, you really nead VERY good diffusion. I would see about building a powerhead reactor. (definately get a second bottle going) There are plans for reactors all over the place and you won't believe the results you get. That looks like 2 80Watt shoplights which is exactly what I have on my 55, another reason to be right on top of the CO2 issue. (in other words, be careful, you're right at 3 WPG, which is perfect for growing most any plants, without VERY healthy plants, it's also perfect for gowing most any algae) I understand you are going for low maint. but in the long run, it will be when compared to cleaning up algae/dead plants. Also, with those HOB's make sure your water is as high as possible to avoid splashing. I really think you got something there man. Just get as much outta that CO2 as you can, get on a good fert. routine and keep on top of it and you won't be dissapointed.
I say ALL of this out of experience, not just off the top of my head. Save up them pennies and get you a pressurized CO2 system! ( ahem...Christmas is right around the corner...cough) Shop around for the parts and it won't be all that bad. Mine wasn't.