Leeches feed by piercing the skin of fish and sucking blood through their specialised mouthparts. Once they are satiated, the parasites can drop off and live on their last meal for a periods of several weeks. They then 'fish' for another fish host by anchoring themselves by one of their two suckers to stones or vegetation, moving the free end around in search of a passing fish. Leeches eggs are small, brown or black, flattened pods which are laid on vegetation, stones or wood debris, often at the water's edge. After hatching, the young leeches must find a fish host within a couple of weeks.
Leeches can cause direct wounds to fish and leave sites for secondary infections.
They have also been implicated in the passing of more serious viral and bacterial infections from fish to fish. Alternatively, the fish could be immersed in a dip tank containing some form of chemical. A large, clean bucket or dustbin makes a convenient treatment tank, but remember to fill the tank and let it stand for a few hours to ensure equilibration of the water temperatures in the pond and the tank. The fish should be placed in the bath for 15-20 minutes, but single fish should be used initially to determine the necessary dip time. If the fish shows signs of distress, remove and transfer it to clean water immediately and dilute the bath.