Hagen does make frameless/seamless tanks and they seem to add a "strengthening bar" whatever that is.
http://www.hagen.com/canada/english/...01034260010101
I thought I also read that frameless tanks use thicker glass. On that website, they seem to supply glass thickness specs for frameless tanks but not the framed ones. I'm not sure why.
Other tanks:
http://www.hagen.com/canada/english/...&PSUBCAT=10301