King fish are the rarest fish you can catch - there are 5 in total. You can catch more than one of the same kind of king fish (not limited to one per save). King fish appear red in the water (not black/silver/yellow) and are much bigger in size, tougher to reel in.

• Casting a line doesn't cost energy, so you can re-cast as much as you want until you get the fish color you want.
  
  
• There will always be 3 fishes that spawn. If a color that you want spawns in, but doesn't get hooked, wait until the hooked fish gets away - the other fish will swim to the hook (don't re-cast).
  
  
• There are foods that increase your fishing proficiency experience gain, gives a % chance of getting an extra catch.
  
  
• The larger fishing proficiency, the larger chance of getting bigger fish to appear and larger pulling force against fish.

*Doesn't matter from which side you fish.

* I re-cast for over 40min in lvl 64 and the fish still didn't spawn, yet in the large lvl 90 lake caught 2 more by normal fishing in under 10min.

After catching the final fish, for me the achievement popped only 3-5 min later.
Some other achievements only unlock after sleeping, so it might also pop the next day.