Water sprite is pretty durable. If it's growing quickly in your tank, I think you'll be OK to plant the rootless cuttings. If it grows slowly, you should float the cuttings until they have a few roots on them. If your plants are already producing roots from some of the nodes, you can cut just below the node and plant the pre-rooted portion. Be sure to cut (manicurist's scissors work well), not break. Not only will you double your plant population, the originals will get lots of bushy new growth after cutting.