ich mag keinen Fisch vs. ich esse gern Fisch
This has a very other meaning than in the original question. It means "I do not like to eat fish."
In case of goldfishes the topic is emotions, in case of fish it is meals.
(Both may be superseeded by context, but to eat goldfishes is extremely seldom.)
Because of the examples in #1 are from a teaching book without further context, I cannot tell what to prefer.
I want to give a short summary of the answers and an addition:
Ich mag keine Goldfische.
Ich mag Goldfische nicht.
Goldfische mag ich nicht.
Usage often as contrast:
Goldfische mag ich nicht, aber Hamster mag ich sehr.
Ich habe Goldfische gern.
has the same meaning as Ich mag Goldfische.
(and the other variants.)
"Gernhaben" (separable verb) is another translation of "to like".