Just want to make sure you are OK on the light issue. As sik80 correctly said, you want an 18-inch tube in T8, and you want a full spectrum or "daylight" type that has a kelvin rating around 6500K. You can get these made by Phillips, Sylvania or GE at many hardware stores for a few dollars. Similar tubes at fish stores will be much more expensive, but basically equal. The "kelvin" number is a measurement of the light's colour, and 6500K is the closest to mid-day sun so it has everything needed.
And on the plants, those are stem plants, and all stem plants just keep growing so they have no "height" as such. When they get to the top (surface) it is good to pull them up, cut off the lower portion and replant the top portion so it will continue to grow. This is a regular task every week or two weeks depending upon how fast they grow. If you don't do this, the plant continues to grow and that means across the surface (not out of the tank) which can be OK in some situations. But when it does this, almost always the lower leaves will fall off, leaving bare stems. The regular "pruning" keeps the plants looking tidy. It's like a regular haircut.