title_body: Espresso machine fish tank smell
upvoted_answer: You've got colonies (of bacteria, and perhaps algae.)
  
  I'd be looking for something a bit stronger than vinegar or citric acid in the sanitizing line, though either of those might be a good place to start, just to clear out any calcium deposits that other things might be using as a growth substrate (attaching to.)
  
  Depending on your preferences, and/or any information you can find about what is recommended for your machine, this might be bleach and water, or one of several choices from the (beer) brewing products such as PBW (powdered brewery wash), Star-san, or Idophor. Star-san (at least) will also attack calcium deposits (it's mostly phosphoric acid.)
  
  Rinse well, many times, when done.