isn't placent 3rd person plural?
It is. That's how the verb "like" works in Latin, Spanish and Italian. It agrees with what is being liked, not with who likes. A more literal translation is "salty fish please me".
You should translate this with "The salty fish please me." Your translation switches up the subject and object of the sentence, which makes understanding what is going on with the verb there difficult, since you don't actually teach any grammar.
Duo does explain placeo in the notes for the Routines skill, but it is not indepth.