Who killed Marcus's Fish?

Poseidon ?

I was going to say that it probably just died, but this Poseidon theory seems more likely.

And for the record, I never thought you were cool. I only thought you were a lesbian.

Maybe he killed it himself

I think it just died of natural causes. My cousin used to have pet fishes until they all died from overfeeding, so that's another possibility.

It's quite possible the fish killed itself. I certainly don't recall Grace sweeping the fishbowl for scissors.

Marcus would be my guess.

The shark obviously.

I don't think anyone killed it, I think it just died. Fish don't really have a long life expectancy. I know someone said that Marcus probably killed it and then tried to commit suicide, but even if that's the case, I don't think he would say that he thinks Luiz did it and then be outside of Luiz's room after cutting his wrist.

I think the fish died, he freaked out upon seeing it because it's like a bad omen to his upcoming day of leaving the facility. He knocked the bowl over, started to write a song, smashes the bowl, takes a shard of glass with the intent of probably harming Luiz as he gave him a hard time and he already thinks he did it, but gets there and probably can't do it and instead decides that he'd rather die in the facility, the place where he feels safe instead of be part of the world where the woman who should have loved him harmed him.