The act of two peoplemaking out with such fervor that it is audible to others around.
Example 1:
(Josh and Lana are making out)
Marlee: Can you hear that?
Amanda: Is that the fish tank?
Example 2:
(Alexander and Kayla were making out the previous night)
Alexander: (sly look) So, I kept hearing a belt buckle last night...
Nathan: Yeah well, I heard you fishtankin' it up! How about that?
Alexander: (holds head down in shame)
Nathan: (ultimate glory)
When it is late at night (around 3am) and your intoxicants have worn off, and you are just a shell wandering or staring aimlessly at walls, like a fish in a tank. Also, to actually fishtank you cannot be aware you are doing it -- that would mean you're coherent.