I strongly suspect that, with there being so many kinds of fish, that their memory spans can't all be the same. Also, there's meant to be an "electrical" short-term memory (e.g. swim forward, watch out for that rock, now the rock is behind me...) and the "chemical" long-term memeory (e.g. when that guy comes, food appears). It could be that a test was done to determine the electrical memory of some fish, and found that they have no idea what minor event just happened, but after a while they cotton on to simple patterns, or learn the position of hiding places and stuff.