It's been several weeks since Harvey's pet fish died a mysterious death in the TMZ newsroom ... and on this All Hallows' Eve ... the creature from the deep returned to wreak havoc!!!
Check out all the cool costumes the TMZ staff rocked into the office today -- Cher and Chaz Bono, a black Justin Bieber ... Fro-Jay Simpson ... and one MASSIVE radish!!!
Click through the gallery -- and then tune in to "TMZ on TV" tonight to see everyone in their full costumed glory.