Goop is a silicone adhesive, as are many caulks and sealers and almost every clear thick glue that isn’t an epoxy. Acetic acid is a chemical byproduct of the curing process.
By the way, “spar varnish” is not used in boats because it is extra-protective from water, but because it is more flexible (and sometimes UV-protective) than regular varnish; this helps when your wood is expanding and contracting. In static applications (like your fish tank) it would be less robust than just an everyday varnish.
