Which simpsons episode is this?

I remember when i was little i watched one of the old episodes, not 80s or anything around the 90s and it was the one where Mr Burns and Lisa were together in the powerplant. I cant remember it that well, but he sang a song (it wasnt the monkey vest one with the puppies he stole either). He was making something from fish, oil or something. Lisa didnt think it was right and she ran off scared because he was freaking her out.. Does anyone else know which one i might be on about ?