Was he thrown into the tank with anything tied to him to make him sink? I mean...they can't have just assumed he didn't know to swim, right?

[Well, not that Filbo knows much about how the aquarium tanks work in the first place...but yeah, not everyone is like a grumpus, sinking like rocks all the way to the bottom of bodies of water, so he's pretty sure that, unless Nameless had something to drag him down, that was kind of an ineffective way to try to kill him]