So, I'm far from an expert here (given this is my first post) but the 5.25% bleach will boost CL 5.25ppm in a 10,000gal pool. Adjusted for 15,000 gal that would be 3.5ppm. That should have landed you around 8.5 but if you're still fighting something it could easily have consumed some of it.

edit - doh, that's what happens when I wait 10 minutes to post - all context gone