Well, "ryba" is feminine, so "on" doesn't seem right here ;) If the given fish is male, then maaaaybe... but frankly, the best option is to just omit the pronoun. The subject was stated a moment earlier, it's still clear in the third sentence what the subject is, you don't have to put any explicitly.
In Nominative, you rather don't use pronouns (on/ona/ono) to refer to anything other than people. Sure, if anything, than for animals, but even if it's not technically wrong, then it's at least strange.
That really did not sound like the usual voicing of "ryba" so I was thrown off. Is it pronounced differently in some sentences than others?
It seems okay to me. When you know how different letters are pronounced in Polish, then you know the pronunciation of 99,9% of words, there are virtually no surprises. It really shouldn't be pronounced differently in different sentences.