It is very difficult to keep cats out of your garden, but do not give up, it can be managed. Cat owners may not understand why gardeners want to prevent cats gaining access to their gardens, but there are justifiable reasons, it is not always that gardeners hate cats. Cats are ferocious hunters and can also be very destructive.
The primary concern of any koi pond keeper, indeed of any fish pond keeper, it to maintain the pond water at a high level of cleanliness. This basically means that you have to keep the nitrate and ammonia levels down. However, the fish will not help in this task, and nor can they, since they live and defecate in their environment, the water. You can accurately say that the live in their own toilet bowl.
Have you ever heard about clothing that repels mosquitoes and other flying insects? If not, I am willing to bet that there have been times when you have wished you could get some. Mosquitoes and midges can wreck a holiday or even make you have to get out of your own garden once it begins go get dark.