Subject: Why is it that some fish live in warm waters and others limited to cold?

My detailed question is:  Some fish--such as carp--can live in relatively 
warm water.  Others, such as rainbow trout, are limited to colder waters.  
Considering both the environmental conditions and the fish themselves, how 
can this observation be explained?  I am not sure why these animals from 
the same phylum must live in different temperatures.  This question has to 
do somewhat with homeostasis.

Re: Why is it that some fish live in warm waters and others limited to cold?