"you" actually is emphasized here by the word order (putting "dla ciebie" at the end), which isn't the most natural one - although the fact that the pronoun is a part of a prepositional phrase here makes it not as bad as usual.
I will remove the sentence and create "Nie mam dla ciebie żadnych ryb".
But "ciebie" is the only (singular) option here, you cannot say "dla cię". Those emphasized variants of pronouns are a must after a preposition. Although sometimes there's also a third variant, which always starts with the letter "n"... and then those variants exist only to be used after a preposition.
Yeah, it is complicated :|