Les chats mangent des some a an (plural) poissons.
why is fish not plural? "s" at the ending.
Yes, "fish" is just one of those funny irregular plurals (one fish, two fish). Many animals words have that phenomenon: one moose, two moose; one deer, two deer, etc. The correct translation is "The cats eat some fish." Interestingly enough, because the plural and singular of fish are the same, it's impossible to tell whether you mean "some" as a partitive or a quantifier ("du poisson" vs. "des poissons"). If you really wanted to make the difference clear, you could say, "The cats ate a number of/several fish."