NT Labs Koi Care Formaldehyde is an excellent anti-parasite treatment, particularly when mixed with Malachite.
Formaldehyde is effective against protozoan parasites and flukes.
It is especially useful in treating flukes because of it's ability to infiltrate mucus and reach the parasites that would otherwise be protected from other bath treatments.

When these parasites are suspected apply Formaldehyde at 10ml per 150 gallons (681 litres) of pond water.

IF Combining Add 10 ml of Malachite Green, to every 352 gallons (1600 litres) of pond water and 10 ml Formaldehyde to every 150 gallons (681 litres).

This is a strong one shot treatment, and is thus not suitable for use with those systems that contain sensitive species such as orfe, sterlet, rudd or tench.

Key Features:

7 Day Application for persistent parasite infections

Highly concentrated

Treats white spot, costia, trichodina

Can be mixed with Malachite

Can be used on its own against flukes