the fish just has less memory.
This was dispelled i believe a while ago by non other than the good ole BBC when they conducted tests in which Sir Robert Winston put his hand in a tank which contained a cross section of tropical fish and didn't do anything. The fish just swam around merrily and then he put a scary looking glove on with fangs and big eyes placed his hand in the tank - and again the fish did nothing, he then made attempts at grabbing the fish who then hid in the corner of the tank.
About a week later he put his naked hand back in the tank - the fish did nothing, then he put the scary glove on - the fish all immediately fled to the same spot in the corner of the tank without hesitation, the experiment lasted quite a while until the fish eventually recognized that the hand wasn't trying to grab them with the scary glove on anymore - but they
remembered that this glove would attack them!