Leeches are not among the most common of goldfish parasites, but nevertheless, they are rather nasty, and easily introduced into an existing tank.
Leeches live by latching onto a living host (such as a goldfish) and feasting on the blood of the host, before detaching and dropping away when they are full, and then repeating the process every time that they are hungry.
Understandably, leeches can pose a health hazard to goldfish, both through blood loss and through the risk of infection to the abrasions left behind after they detach from the skin.
Preventing and treating leech infestations
What causes leeches to infest a tank?
There are a whole variety of ways in which leeches or leech eggs can be introduced into the goldfish tank, and one of the most common of these is by adding plants or even fish to your tank that have been caught in the wild.
You might also find in rare cases that store bought fish or plants may harbor leeches, which is another reason why quarantining and inspecting new additions to your tank is so important.
Some forms of live food can also carry leech eggs with them, so you must ensure that you only use live food that comes from a reputable supplier.
What are the symptoms of leeches in a goldfish tank?
Leeches are relatively easy to recognize and diagnose in your fish, as they are large enough to be spotted with the naked eye!
They look rather like worms, only larger, and once they are well fed, they can appear quite fat. Look for them around the body, fins or gills of your fish, or swimming through the water.
Treating leech infestations
Because leeches are reasonably large, it is often tempting to try to physically remove them from your fish using pliers or your bare hands. However, this is not an appropriate way to remove them, as the chances are that you will kill the leech while their head is still embedded within your fish, leading to a serious risk of infection.
The best way to remove a leech from your fish directly is by bathing them in a 2.5% aquarium salt solution for 10-15 minutes, which should cause the leech to loosen their hold on the fish, and either drop away, or be easy to pick off.
If you have a large leech infestation within your tank or the potential of leech eggs in the substrate, you can treat the entire tank by means of a Trichlorfan solution at the rate of .25mg per liter of water.
If you intend to treat your entire tank then you may need to remove your plants to check for leeches or eggs hiding around the roots too, which may be able to survive whole-tank treatment.