I used one 10 oz tube of Dow 832 black on my 125G. I removed all interior silicone on the 8 inside surfaces and applied a bead equivalent to the glass thickness.
I bought an extra tube, just in case, but didn't need it. Just remember, this stuff does have a shelf life, so if you can't use it before the expiration date (or within a reasonable amount of time), you will just be wasting money.
BTW, I paid $9.17 per cartridge (Sept 2009) from Mcmaster Carr.com and could not believe it when they delivered it the same day, right to my home. I forgot they had a local distributor in Aurora, which is not too distant from my home.