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