"The fish is his dinner."
Translation:De vis is zijn avondeten.
simply because it's "de vis". I'm afraid in most cases you need to learn this by heart.
Animals though are normally "de", since they are either male or female opposed to the neutral "het". De vis, de vogel, de beer, de olifant, de tijger, etc
It's only "het", when you make it into a small animal: het visje, het vogeltje, het beertje, etc