Depending on the type of fish and the type of food, I frequently will turn off or cut the intensity of the filtration system when feeding. Holy Pavlov Batman! Does the same thing as any of the others.
This isn't really training though, it's an exploitation of a natural instinct coupled with pattern recognition. Animals are built to adapt slightly to their environment, repeated application of an unrelated stimulus with something there is an instinctual response for will result in an association between the two being formed. It's not the same as teaching the animal anything, it's simply adding to stimulus recognition.
Now... if someone can get a fish to do something a fish would never otherwise do with either no reward or no utilization of an associated instinct (or a snake for that matter) then there's some training involved. Not gonna happen anytime soon though.
Instinctual isn't always the same as stupid and procedural memory can frequently produce much more desireable results than declarative memory from the standpoint of an individual animal.
"Genes, Like Leibnitz's monads, have no windows; the higher properties of life are emergent... And once assembled, organisms have no windows." - Edward Wilson, Sociobiology