Settlers Cities and Knights + Fish: Cost of a commodity
You cannot buy commodities using fish.

http://download.catan.com/en/download/?combination-Fishermen_Cities-and-Knights.pdf


  
  If you discard 3 fish, you may draw one card from the hand of one of your opponents  (Resource or Commodity Card). 
  If you discard 4 fish, you may take resource card but not a commodity. 
  The following applies also for the combination with Cities &amp; Knights: Fish are no resources  – and no commodities. This implies
  that fish are never affected by Progress Cards,  particularly not by
  the yellow Progress Cards.