We have used bacteria eating enzymes, but they did not clear moss or algae. It is also a different enzyme than what you would buy in a general store. You need to check with a fish/aquaculture facility to ensure that the enzymes will not hurt the swans. You can use an above or below water aerator/de-icer which will help with oxygen in the pond. Ask the fish/aquaculture specialist about ion or UV type filter systems that may help control bacteria counts.

You must have proper water flow/oxygenation or the swans can die from botulism caused by anaerobic (non-oxygen) bacteria. The Regal Swan