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.
When you are sitting back in a group setting or party and just observing what is happening and watchingthe drama unfold.
Do you want to get to the party later and do some fishtanking? I heard there might be some drama that goes down. Perfect opportunity for us to fishtank.