But by the 5th cycle (the 5th time they grow to hit the water surface) it is often the time to trim and replant again.
For most stem plants, you can trim in the second cycle (after having replanted the healthy tops the first time) and maybe the 3rd cycle. But by the 5th cycle (the 5th time they grow to hit the water surface) it is often the time to trim and replant again.