Water Sprite grows like CRAZY when planted. Just let it float for a while and develop some roots, then plant it in the substrate.
The fronds will get bigger and bigger as the plant ages. Eventually the fronds will blacken and plantlets will form on it. This is when I usually just clip the whole frond at the base and pitch it.
After a bit (in my aquarium, anyway), the fronds got too tall for my 29gallon, so I just pulled up the whole plant and traded it back to the LFS. Fortunately, I kept a couple of the plantlets mentioned earlier. I've been through the process with a couple of the plantlets too. They just get big FAST.
The darn things suck up nutrients like mad, so keep up with the ferts!
Your mileage may vary.