I recently learned that Guardians killed by a player have a chance to drop an extra fish, in addition to normal raw cod. This additional fish apparently has a 60% chance to be a raw cod, 25% chance to be raw salmon, 13% pufferfish, and 2% tropical fish. However, I observed that this never seems to happen. I killed around 300 Guardians with a Looting III sword, and I never got any raw fish other than cod. 

How to replicate:

-Create a small chamber, at least 3 blocks high, with a dispenser above it. 

-Put about 1 stack of Guardian spawn eggs in the dispenser.

-Create a redstone loop that will trigger the dispenser constantly.

-Let it run for a minute

-Give yourself a sword enchanted with Looting III (optional, but highly reccomended)

-Use commands to give yourself the Strength status effect, at a potency that would enable you to kill the Guardians in one hit.

-Kill all the Guardians that spawned, and pick up all their drops.

-Repeat all the above steps a few times.

Expected result:

By sheer luck, you should have at least one raw fish that isn't cod.

Actual result:

There are absolutely zero Raw Salmon, Pufferfish, or Tropical fish in your inventory.