First thing to know that it is strongly advised not to use fed leeches for your treatment purposes. If the leech is used for one persons treatment, it takes 3-6 months to digest the blood. Therefore it contains a lot of risk for a leech to contain bacterias etc. even though medicinal leeches contain anti bacterial enzymes. There is still a lot of risk for a leech to pass bacterias and toxics to another person if applied.
In their natural habitat, it is known that leeches feed themselves with frogs, fishes and any other animals they can hatch on. Blood is what they need basically.
One common mistake is to use leeches only to unclog the veins. Unlikely most effective ability of a leeches is the enzymes they have and pass it upon being fed such as hirudin.
Buy leech only sell unfed, sterilized and quarantined leeches.