Stem plants grow continuously, and toward the light. As they are fast growing plants, they need brighter light than some other plants do, and more nutrients to feed the growth. The longer the stems grow, the more likely the leaves near the lower portions that are farthest from the light will yellow and die off, this is normal. To keep them bushy, you pull up the stems, trim off the lower portions (or cut off the upper portion, depending how you see it) and plant the upper cut portions in the substrate. When they get growing again you repeat as needed. If you just let the stems grow, the bottom portions will be completely bare and the stems will grow trailing across the surface.