There are ten koi fish in my fish tank. When I feed them, I will buy a catty of small fish and put them in the fish tank. Koi fish caught them and ate them. Is it okay?

Not good, and it is very bad. Fish tank culture (no matter what kind) should not be fed more. If the fish can't eat for a while, the fish food will rot and affect the water quality. Even if you feed small live fish, don't put too much into it at once, because koi fish can't finish it, and the remaining small fish will consume oxygen in the fish tank. You also need metabolism. Even if you have a pump to supply oxygen and discharge sewage, it is better not to feed more. It's just a meeting.