Don't bunch them together and plant them, spread them out to 1 stem and plant them 1"+ apart, that will help them get more light and won't get their stem smush. I had trouble growing Anacharis, they need more light then most people think, not as much as some red plants, but they need enough, and an aquarium light strip just won't make the cut. I live in texas and these thing taking over the river because of the sunlight they receive. Another plant you might like is Hygrophila Polysperma, its easier to grow then anacharis.