Movement of this parasite is by means of looping like a caterpillar. Leeches feed on blood and can also be bearers of blood parasites. They lay there eggs in cocoons on the pond bottom, plants, stones, etc. The leech is not an obligatory parasite and may leave their host for long periods of time, as they can survive for prolonged periods without feeding. The greatest threat to the fish from the leech is caused by moving from fish to fish whilst sucking blood, with a great risk of spreading any virus diseases present to all the fish in the pond.
Leeches are common in planted ponds and are less likely in non planted ponds.
They can infect all parts of the fish including the mouth and gill areas, and this parasite is extremely difficult to eradicate. The life cycle of Leeches can take from a month to more than a year depending on the species and temperatures of the water. Studying temperatures at which the parasites life cycle is speeded up so as to pitch the use of chemicals accurately is one option of eradicating them, the other alternative is to completely strip down the whole system and allow it to dry out. Chilodonella is another important ciliate ectoparasite on a wide range of freshwater fish. It has a flattened ovoid shape and is covered by rows of cilia which move it in a steady gliding manner over the epithelial cells on which it feeds.