"The fish do not eat the cats."
in all latin languages, nouns and adjectives have diferent forms acording to the gender and number ... in your example i gatti means the cats (males) and le gatte means the cats (females) as il gatto means the cat (male, one male) and la gatta means the cat (female, one female) ... for example, in French the cat as two forms: le chat (male) and la chatte (female) ... and so on you have to learn by heart ... is it ok ?
The word "Fish" can be both plural or singular. As user sdbondi pointed above, since they use "DO" instead of "DOES", it means that "The Fish" is in the plural form: "The Fish", a.k.a. "They", DO. If it was singular, it should be The Fish DOES (doesn't). Tricky, but legitimate.