Pondering a Larger DIY Tank -- Questions

[dbosman]

How about four 48" wide tanks, such as 75 gallons?
Or a couple of 72" wide tanks, such as 125-150 gallons?

Your real limit on a long tank will be the length of glass your local glass supply shop can order in. That will be some standard length at a reasonable price, or a custom length at some not very reasonable price.
Sounds like fun for someone who can afford it. I look forward to seeing pictures in a couple of years.

[Pulmonatus]

You'd have great oxygen dissolution with that much surface area! Live plants would really thrive if you can get the right light hood over a behemoth like that. If you can, I'd suspend the lights from the ceiling, rather than using a glass cover, to maximize the benefit from surface area. I would also plan on some shrimp to help keep all of that gravel clean for you. I've always been a fan of long and shallow tanks, but then again, all of the interesting species are at the bottom of the water column, right?