I'd venture to guess that the nutrient load that the swans are putting into the water is the culprit. The algae cannot keep up with the nutrients, and you are barely staying ahead of the curve with the water changes that you are doing. Somethinng that will utilize the nutrients would help greatly. Lots of plants, a huge biofilter, etc.
Swans are big birds, when they're in the water, they are basically swimming in their own toilet. Take a water sample to an aquarium shop to have them test the water. Do this every day or every couple of days starting when you change the water until a day after it turns black. You'd be suprised at what the results would show.
