You will have to help the system from getting out of control in order for the fish (and ducks) to remain healthy. You can only add so much nutrients (duck food, duck poop, fish food, fish poo, bugs that fall in, plant matter, etc) before the system overloads and infections take over. Now, fish survive just fine in nutrient laden systems, in fact, certain levels of nutrients are essential for good inhabitant health. It will be up to you determine when intervention is required. Most small aquatic ecosystems (small garden ponds or aquariums) require human intervention to help maintain a healthy environment. This intervention will consist of 10 to 20% water change on weekly or monthly basis. Remove 10 to 20% of the water and replace with fresh water (de-chlorinated if from city water supply) and of the same temperature of the fountain water. The frequency will be up to you to determine. There are water quality test kits that can be used to monitor the water conditions which, in turn, would help determine how often water changes would be needed. I would be checking for PH and ammonia, in particular.

Fish on!,
Noel