Interesting choice, I have heard a rumour that leaches will relish a feeding of beef heart and possibly other organ meats. It is also quite common for leeches to not eat for extended periods of time I believe up to a month or a few months. You should be able to procure beef heart from your local grocer. look for other things such as gizzards or chicken livers if you can't find the former. They aren't as voracious a predator during their adult phase of life don't be discouraged by it's lack of interest in your arm. In nature blood only makes up a portion of the leeches diet. leeches are primarily nocturnal predators so try feeding just before lights out and make sure it has a place to hide in the aquarium.