"Peter likes fish very much."
Translation:पीटर को मछली बहुत पसंद है।
As stated in the Food lesson
For other nouns, the particle को (to) is used. Indirect objects are placed after subjects in Hindi. For example:
राज मुझे अपना केला देता है - Raj gives me his banana. (देना – to give)
मैं उसे एक बिल्ली देता हूँ - I give him a cat.
राज जूलिया को दूध देता है - Raj gives Julia milk.
वह नेहा को चाय देती है - She gives Neha tea.
So, if it is a noun in the dative case, you would use it.