We know a good deal of blood and destruction occurred in the onsen. It is quite possible the murder took place there, although the real Cosmos was found in the aquarium, and the false one also had its head bashed in. So perhaps the false Cosmos was killed in the onsen and the real one in the aquarium, but for what reason I am uncertain.

[even though they aren't bad yet.]

In any event, at some point in the night, the aquarium glass was bashed in. A bloody handprint was left on the glass. The octopus or a body was dragged into the bar. Three drinks were had - the impulse control drink, the persuasion drink, and a glass of water.

I would guess that occurred before whatever happened in the onsen.