Boyfriends that cheat on their girlfriends with nasty bitches and catch a repulsive infection that makes their penis smell likefish. Thus, the male being called Fishco.
Sarah: Dude, my boyfriends dick smells likefish...
Laura: Oh no, he's a Fishco!
To 'knock over the fishbowl' is a euphamism for failing a 'girlfriend test', where your girlfriend sets up a test to ensure that you are committed to the relationship.
Alice invited me over to her apartment. Turns out my girlfriend was waiting there... Way to knock over the fishbowl.