Because of the larger angle of the glass panels in relation to each other, it is natural that the bead does not look as thick as on the right angle corners of a rectangular tank. In the physics of a silicone bonded joint, it's my understanding that the silicone that is directly between the facing edges is ~80% of the holding force. Yes, it's a small surface that does most of the holding. The remainder of the bead surrounds that small area for additional strength, but mostly for protection of the main strength zone. It's interesting that we commonly think of silicone as rubber which stretches, but in that minute distance between glass edges, the "stretch" is extremely minute.
With used tanks, I check very carefully for missing and frayed silicone on the edges of the bead. Some fraying is acceptable. Unless the tank was put together with black silicone, I also check to be sure there is no color difference in the center of the clear bead along the entire length of all joints. A color difference may indicate that the glass edges have pulled apart and broken the silicone bond. This bond usually starts small, just an inch or two and may not leak because of the surrounding bead, but with the constant force of water pressure, the bond may continue releasing over time, leading to a catastrophe.
Speaking of color, if the silicone has a yellowish or greenish tint, that tank is too old or was left out in the sun and could easily fail. If the silicone has a bluish tint, that's okay. The blue is just a freshwater medication dye that absorbed into the silicone. It's not a problem. Old yellowish silicone may create a greenish color in the presence of that bluish dye (yellow + blue = green).
Hope this helps.
Aloha,
Mark
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