Leeches are common segmented worms called annelids that have long been viewed as parasites, attaching themselves to the skin and bloating themselves on your blood. Hence the expression, "What a leech." But according to the National Health Museum, these little annelids, found in environments as diverse as large rivers and marshy swamps, have been used by medical professionals for centuries to treat problems from muscle tension to heart disease.
Obtain permission from the U.S. Food and Drug Administration if you plan to sell your leeches for medical use. According to WebMD, the FDA approved in 2004 the application of a French company called Ricarimpex to sell medical grade leeches as "medical devices." Although the company has bred leeches for a century and a half, it wanted to begin selling the creatures to American medical facilities. See the second link in the Resources section for information about applying for FDA certification.
Purchase medical-grade leeches from a science supply store or online source to begin your own colony (see the first link in the Resources section). Only the breed Hirudo medicinalis and about a dozen other species of leeches qualify as medical-grade leeches.
Fill your aquarium 3/4 full with spring water, which doesn't have harmful additives like chlorine.
Place aquatic plants and other natural materials like stones or logs to give leeches hiding places.
Feed your leeches sparingly, from a range of foods recommended by your retailer. Feed them small meals like snails, live worms or aquatic insects on a weekly basis, larger meals like frogs or small turtles every month or two. According to Dailypuppy.com, a colony of small leeches can live off a live frog or turtle for as long as six months.
Change 1/4 to 1/2 of the water a few times a month. This will keep the tank clean and avoid shocking the leeches by drastically changing the chemical composition of their habitat.
TL;DR (Too Long; Didn't Read)
Use leeches as bait, if medical uses are out of the question.