Floating plants don't know where "up" is, so they tend to grow in a gnarled form, twisting, looping around, etc. Then when you finally plant them in the substrate it takes quite awhile for them to begin to grow up instead of in random directions.
You normally plant stem plants with no roots on them. They are just cuttings. You should poke them down into the substrate as deep as you can, preferably at an angle, so they can't float back out. This is easy with long tweezers.