Wow, this will be a beauty when it's all together. What are you using for your front window? I like your ABS plastic idea... I used epoxy and I wound up doing lots of coats just to assure myself I'd got every nook, but your way looks pretty tough to screw up!
I like your stand - that think will hold a locomotive! The only thing I'll say now is that you will regret having the tank so high :-S. You're absolutely right, it looks fantastic, but I did and thought exactly the same thing with my smaller 5.5'x2'x3' tank. Climbing a ladder and trying to get something off the bottom of the tank by leaning over the side is a PAIN. At least in your tank for aquascaping you can drain some water and get into the tank - if you're willing to do the acrobatics to get past the ceiling.
This will look really nice - if I had your space and that spare tank for filtration, I'd consider a bead filter - they are really easy to clean if you set up the plumbing properly - you just run water backwards through the filter rapidly to stir up the beads and dislodge the dirt - the dirty water goes down the drain with all the buildup, and you never even get your hands wet. The settling tank will get rid of all the heavy dirt, but things like bits of plant tend to float - I have that problem in my sump because I use sock filters. The plat matter just floats around int eh sock without ever sticking to the side. Then when the sock plugs up enough for water to overflow it, all the leaves and crud winds up in my drip plate.
. I read somewhere that the pond liner has the tendency to squeeze in between cracks and break so I put in the silicone.
I just used the cheaper DAP.
