What soapy said. I'm no tank builder either, but from the looks of it the 'bubble' is in the layer of silicone that is applied after the glass panes are actually bonded together. If not mistaken, a layer of silicone is added at all seams after the tank is assembled & smoothed out to give that 'finished' look. If that tank isn't leaking right now, I wouldn't sweat it. Clearly the quality control person was having an off day. But this is the internet after all....
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