Hello; I had a seam separate on a 55 gallon in the past. The tank had been in use for several years. I do not know what the seam looked like prior to failure. The tank was last set up in for a few months front of a large window with heavy curtains to block the light but I suspect there was a lot of heat. The tank failed on a hot day.
To the question of how to judge a seam. I suspect that a seam of silicone with no tiny bubbles at all may be best, but have seen some bubbles in what have proven to be sound seams. There is a look silicone gets when glass pieces that are bonded are pulled apart. I know what it looks like but am not sure how to describe it. I have bonded glass pieces and later separated them and noted a distinct apperance. Perhaps a couple of pieces of spare glass can be bonded and allowed to set up for a week or two and then pulled apart in order to view the result. Do not bond too much surface as it may be extremely difficult to get them apart.
I have been in the practice of adding clear plastic corners to my all glass tanks. I have found some that are for protecting the outside corners of wall paper that work well with a little trimming. I attach the plastic corners to the outside of my tanks with clear silicone. My goal has been to protect the glass edges and prevent chips but it seems reasonable to think that they will add some strength to the tank seams as well. Perhaps a stronger material can be bonded to the outside corners that will add even more strength.
I also think that resealing the inside seams of a tank with silicone adds some extra bonding strength, thou perhaps not a great deal, in addition to sealing for leaks.
