"The cats eat the fish."
Translation:Kediler balığı yer.
You don't need to repeat the plural suffix when the noun already has it, and in fact I think I read one of the mods saying that actually the only time it's right to repeat the plural suffix is when the subject of the verb is people (e.g. kızlar balığı yerler would be okay according to this rule).
Uhm, in English "fish" is singular and plural of one kind, but "fishes" is plural when you refer to different kinds of fish... ;-) http://dict.leo.org/ende/index_de.html#/search=fishes&searchLoc=0&resultOrder=basic&multiwordShowSingle=on
The -i on your example of kedileri would be the accusative suffix that you can use on the object of a sentence. In kediler balığı yer kediler is instead the subject of the sentence, so you can't use the accusative suffix. So kediler here could mean "cats", could mean " the cats", but that's not a determination you can make from the grammar with the subject of a sentence in Turkish, only with objects.