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.
"We have to get Kevin home -- he's totally fishtanking."
(verb) the act of removing the lid on the top of someone's top bowl of their toilet and deficating in it. Followed by placing the top bowl lid back on. Fish tanking is used so that someone can scrub their toilet but they won't be able to get rid of the smell because the defication is in the top part of the toilet.
John fishtanked Scott's apartment and Scott could never get rid of the smell, because the terd was in the top part of the toilet.
When two people are making out
intensely enough and for such a duration that they make a considerable amount of noise.
This often irritates the unlucky individuals around them, who must either tolerate the noise or awkwardly tell them to "shut the fuck up
Girl: "Hey, I totally know about how you you two were hooking up in the closet last weekend."
Guy: "How'd you find out?"
Girl: "Because you two were totally fish tanking!"
its like teabagging
but before the testicals go in the partners mouth, they take a drink so their mouth is full of liquid.
"yeah it was so great when i fish tanked lorn, it was rather exellent" i love "fish tanking"