Cut the stems down regularly and replant the cuttings if you want, this allows light to get to the bottom to keep them growing. Also get bottle of flourish comprehensive (or an equivalent) to continue to give ferts to the plants, use this with the excel as per bottle instructions for best results. Growing plants is all about balance, picture a triangle with light, fertilizer and co2. Keep the balance right and you will be away laughing. If you do not have a light timmer, I suggest you get one. They are a planted tanks best friend.