First off, I'm not sure if lilies are self-fertilizing, or if they need a second plant to produce viable seeds. If one plant can produce viable seeds by itself, I would just let nature take its course. Just wait and see if any new lily plants grow in your tank. If you get more growth lateral to the base bulb, that could signal bulb reproduction. If you're not going to move the plants to a new tank, I would just let it be, I love natural growth in a tank!