Been a long time since chemistry classes- but the reduction chain for hydantions looks problematic. Like waterbear said, the cheap test may work, and will give us all some data for future use.
I've been trying to reach an old buddy of mine, who works at water treatment plant (with hydantion type slow-release chlorine that waterbear mentioned) to see what they do to remove it after treating the water system- may give us a clue to pools, or may only be practical in industrial environment... problem is I've been out of touch with them for 20 years, since I joined the Navy!