Formaldehyde

Formaldehyde 30% is effective against protozoan parasites and flukes. It is especially useful in treating flukes because of it's ability to penetrate mucus and reach the parasites that would otherwise be protected from other bath treatments.
Caution:  Formaldehyde is a hazardous chemical and should be handled with great care, please read the warnings on the label carefully. 250 ml treats 17048 litres (3750 gallons)

Can be used in Ponds with low level salt ( 1 to 1.5 oz / gallon salt ) with no detrimental side effects

see Learning Zone : Medicinal Treatment Rules