It's not a preposition, but a separable verb: zusehen To look at. I suppose we could say the "zu" is the same as English "at", although the word order is different.

German verbs are often composed of two parts. I found this very comprehensive list of such verbs https://www.verblisten.de/listen/verben/vorsilbe/zusammengesetzt/ueberblick.html. I obviously didn't check all of them but as a general rule you can assume that they would be split up when conjugated. The basic pattern is like this: <prefix><verb-part-in-infinitive> becomes: <pronoun> <conjugated-verb> <prefix> The object is depending on the prefix either dative or accusative. I your example that would be Infinitive: <zu><sehen> (to watch) becomes: <Ich> <sehe> <dem Fisch> <zu> (Dative) Infinitive: <an><sehen> (to look at) becomes: <Ich> <sehe> <den Fisch> <an> (Accusative)

yes, it is "zusehen", for details see my answer below. Don Holgo is right that the separation rules I describe do not apply to all verbs in the list.

I am confused, with differents answers, I believe that is separable verb zusehen. is it right?

I think the separable German verb "zusehen" often implies "to stand by" in addition to meaning "to watch", so quite often "etw. (dative) zusehen" means "to stand by and watch sth." e.g. https://www.jurion.de/gesetze/eu/51996ac1094/ DE-DE Die Europäische Union darf dieser Entwicklung sicherlich nicht untätig zusehen. -> EN-GB Clearly, the European Union cannot remain indifferent to these developments. But here "Die Katze sieht dem Fisch zu." might simply mean "The cat stares at the fish." Hope this helps. Cheers & Happy New Year Stor