I got a leech with plants. He ate snails and was much fun. I kept him in a jar, and not with my fish. I think there were more in the tank, but they were eaten. Most leeches are predators and NOT bloodsucking parasites.
But they are kinda gross...neat in a jar, gross running wild in your tank. I kept mine on my desk by my computer. He only lasted a few months but I enjoyed him. (Please replace all masculine pronouns with hermaphroditic ones.)
Give the fish time to eat them, rinse the sink out really well after cleaning your filter, check the fish periodically for sucking leeches (who knows WHAT you actually got?), and you'll probably be OK.