Yes it is from the Epsom salt and that is ok.. You CAN mix epsom salt in the water for duck tummy problems... Here is the link from liveducks.com
Botulism
Symptoms: Loss of muscular control of legs, wings and neck, sometimes called limberneck. Birds are unable to swallow.
Cause: Toxins produced by Clostridia bacteria in decaying animal and vegetable waste. Avoid problems by keeping ducks out of muddy/dirty areas, and stagnant water, especially in hot weather. Bacteria multiply rapidly in warmer temperatures with anaerobic conditions (no oxygen).
Treatment: Give affected birds fresh drinking water. If necessary, introduce water into the mouth and throat. A crop tube could be used with the advice of a vet. Add Epsom salts (magnesium sulphate) to the water. Recommended amounts vary from 1 tablespoon in one cup of water to 1 ounce per 50 fluid oz (2½ pints) of water.