Re: why do goldfish have such a short memory?

There is no easy answer to why fish like goldfish have memories that appear 
to be so much poorer than our own.  The parts of our brain that provide us 
with much of our sense of consciousness and meory lie in the cererbum.  
This region of our forebrain has greatly increased in size and complexity 
throughout our evolution in comparison to other parts of our brain.  The 
fish forebrain is not nearly as large in relation to the other parts the 
fish brain.  Furthermore, it has been proposed that whereas tasks such as 
pain perception are performed in the cerebrum in the mammalian brain, the 
same tasks are performed in the brain stem of the fish brain.  It is 
therfore difficult to compare how and why humans and fish perceive, think, 
and remember, because many of these mental events may be fundamentally 
different between the two species.  Memory is an especially tough one, 
because not much is known about how memory occurs in either species.
Furthermore, domesticated fish such as goldfish are probably relatively 
inbred. This means that they are not as "bright" as a fish in the wild who 
can mate with any fish it chooses. This probably has as much to do with 
memory span as whether the fish is an predator or prey.

Fish certainly can have long memories! Paradise fish (Macropodus 
opercularis) can have an aggressive encounter with another fish, and then 
be able to recognize that particular opponent at least a day after the 
initial event. A fish called the Arctic char can tell its brothers' and 
sisters' smell apart from unrelated fish of the same species. They can 
remember their siblings' smell even after several days of isolation. Some 
reef fish can remember where they found a good food source and return there 
day after day. Other fish (Spinachia spinachia, and Gassterosteus 
aculeatus) can encounter a new type of prey, become efficient at feeding on 
that particular food source, be separated from that food source for 8-25 
days, and then be able to remember and use the technique they previously 
developed. Salmon, during mating season, return to the very stream they 
were born, by their sense of smell - this can span years! So fish can 
definitely remember things for extended periods of time using multiple 
senses. The length of time any particular fish can remember something and 
which senses it uses to do so, will vary between species and habitat. 

I hope this helps!