In the course of putting together your aquarium environment, you will introduce a variety of living organisms: plants, invertebrates, “living” rock and other media for beneficial bacteria, and of course fish. However, as in any environment, parasites may find their way in. One common parasite that can be damaging to your aquarium environment and harmful to your fish is the leech.
A leech is an annelid, or segmented worm; it is flattened, with a sucker on both the tail and the mouth. Leeches propel themselves forward by using both suckers, producing a motion similar to that of an inchworm. Most live in water; there are freshwater, saltwater, and terrestrial varieties. Many leeches, given the opportunity, will attach themselves to humans and other creatures, sucking the host creature’s blood. It was common medical practice in the Middle Ages to “bleed” a patient by attaching a leech to the patient’s body and allowing the leech to drink his fill. Medieval medicine wrongly held that the body was composed of four principal fluids, or “humors,” and that an imbalance among these humors was the main cause of disease. Blood was one of the humors and was often thought to be in excess; thus, bleeding a patient helped bring the level of blood back into balance with the other humors.
Leeches are usually easy to spot in your aquarium: it’s unlikely you’ll see any of the larger varieties, but freshwater leeches that might find their way into your aquarium can still be up to 2 inches long. Leeches swim in an undulating motion. If they are attached to a fish, they may be heart-shaped, as they curl up onto your fish’s body, fins, or gills. The odd leech bite isn’t going to harm your fish in any major way, but an infestation can cause major problems, causing fish to become listless and thin. Leech bites can render fish susceptible to other diseases; open wounds can become infected. And a leech can transfer microbial diseases from one fish to another.
Leeches might first enter your tank via live food that comes from leech-infested waters. If you rely on live food that you grow yourself, you can greatly reduce the chance of a leech infestation. Leeches can also come via new fish that you introduce, or new plant life. Be sure to inspect new creatures or plants thoroughly; if you’re concerned, you can keep new fish quarantined for a period of time before allowing them to mix with your existing specimens. And you can place new plant life in a potassium permanganate solution, 5 mg/l, for about an hour before introducing them to your tank. The potassium permanganate will kill any leeches that might be hiding in the foliage.
If you find a leech attached to your fish, don’t be tempted to remove it with a pair of tweezers (or pliers!); leeches burrow themselves into your fish’s body to draw blood, and forcible removal can seriously harm your fish. Instead, bathe your fish for about fifteen minutes in a 2.5 percent saline solution. This should cause the leech to let go, or to loosen its grip such that you can ease it out without leaving any leech parts inside your fish. If your fish are sensitive to salt -- for instance, certain species of catfish -- then saline solution will not work; you may need to medicate your entire tank by adding 1 milligram of trichlorofon for every gallon of water in your tank. The trichlorofon may be harmful to some plants, so it would be best to remove the plants first and treat them with potassium permanganate, as described above.
Although leeches are not everyone’s idea of an ideal aquarium pet, they are interesting creatures in their own right, and some people do keep them, sequestered in their own small aquarium of course. They are easy to keep and don’t demand much food: a small piece of meat or an earthworm every week is usually enough to keep a leech happy. They are quick to seek out food sources, however, and will respond to your finger rubbing along the outside of the tank as well as to shadows passing overhead. If you keep substrate in your leech tank, the leeches tend to burrow; they are also excellent scavengers. Most leeches are freshwater, but because there are also marine varieties, be sure you know what kind of water your leech will need.
