Yeah, eartheaters do need earth to eat. I'd be willing to bet 100lbs isn't enough for that massive footprint though. I have I think 80 lbs PFS spread between a 4x2 and a 6x1.5 footprint and it's pretty thin (though I like it that way, easier to clean). That's 17sqft. Yours is more than twice that (god, my geos would be so jealous if they knew...)
Good call on the removable background. That's my one hesitation about painting my acrylic. I know I want black and a nice dark tank, but I don't know that I want it to be permanent...
I'm guessing that having returns on that other side would be great for circulation. Aim them low and stuff on the overflow side high. Would look better than plugging the holes. And being that it's in the living room then you could hide them with a removable end piece and maybe some nice moulding. Sorry, not to get pushy or anything, it's just what
I'd do. Never hurts to impress guests even if the fish are asleep. And you can accomplish a lot with trim, too... it's not all that complicated.