This is the exact snakes I kept having in my pond last year. I have not found any efficient way of keeping them out. The moth balls are merely a mild deterrent. Once they know there is a food source there, they don't mind sliding right over the moth balls. Also these are only good if you have no other animals. I have dogs and cats and these are toxic to them also. I had two of these snakes last year and they ate all but 3 of my fish. And the fish were pretty big. I also have lots of ribbon snakes every year, but these are too small to eat my fish, so I don't mind these. The only thing I have found that works is my poodle, she loves chasing and killing the snakes. She thinks it is a game. I never like killing anything that is not poisonous, but when they are eating my fish then I don't care if she goes after them. She usually gets them within a couple of days, but I do lose fish by then. Sorry I have no good answer.