I have a couple of used 75's that are 19-20 years old. Both were stored dey for a few years, but hold water w/out any problems now. They don't even have center braces; the glass on the older tanks is thick enough not to need them.
I have an old 55 gallon with a slate bottom that still doesnt leak with original seals.its gotta be old..the glass has a green tint no matter how clean i get it though..Its my feeder convict breeder tank