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 getKevin home -- he's totally fishtanking."
When you are sitting back in a group setting or party and just observing what is happening and watching the 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. Perfectopportunity for us to fishtank.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.