NT Labs Koi Care Malachite is an effective treatment for external parasite and fungus infections. When used together with formaldehyde it's activity is enhanced which means that less malachite can be used. High dose or frequently repeating dosing can damage the gills of the fish. If the required effect has not been achieved after two treatments take further advice to establish the real cause of the problem. Please note the warnings on the label and handle the product with care. 250 ml treats 20000 litres (4400 gallons) Also available in 500ml - See model YR118