That chip or "shell" in the corner might or might not be a problem in itself, depending upon how deep into the glass it goes. But that entire vertical seam seems to be giving way. Is that what you are referring to as "working up"?
I can't help but think that this is being caused by a tank that is not evenly supported all the way around. If the surface upon which it rests is not perfectly flat, i.e. if it is twisted in any way or has any imperfections or debris that apply pressure to the tank, that seam might be subjected to a lot of twisting force that might cause that damage.
I'm no expert, just a guy who has built...and wrecked...a lot of tanks over the years. I must say that if that tank were in my house I would be draining it sooner rather than later. I don't like the looks of that seam at all.