It's in the silicone that sandwiches the two panes of glass together. I understood this as the "primary seal". I have no idea how to build a tank, but a buddy of mine does quite a bit of it. He says not to worry but man... I am not loaded and would be so disappointed to have a tank explode and lose a bunch of fish and corals.
That other bubble you see is actually in the water caused by powerhead.. We are looking at the lengthwise skinny bubble in the seam.
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