Where is the word "one" in the sentence, or do you not require the word one in the sentence?
correct. Not in Czech. There really is no equivalent of the word 'one' (outside math of course) and if anything, you would have to repeat 'ryba' -in this case 'rybu' if you wanted to point out that it, too, is a fish.
Helpful to read these comments, because mine was corrected with this English statement "the big fish eats the small" which makes no sense. Even if there is no word for "one" in Czech in this context, in English it is important in order to understand the translation.
From what I've seen sprinkled throughout the course, you can use "jíst" with animals as well as with people. But it would be considered rude to use "žrát" with people... although one actually may intend to be rude in that way on occasion... :-)
I eat = žeru; you eat = žereš; he/she eats = žere; we eat = žereme; you eat = žerete; they eat = žerou.