Your dose of 6.5 gallons is right. While you are fighting this, as often as you can test and add more bleach the better. There is no such thing as doing it too often. Every time you test, add enough bleach to get back to 15. You could stand to bump the CYA up a little if your reading of 30 is right. Maybe add another 2 lbs. of CYA. Put it in an old sock and hang it in front of a return jet. Give it a squeeze every once in awhile to help it dissolve faster. That should add another 10ppm of CYA for you. You want to continue shocking until you can go from sundown one day until sunup the next day without losing more than 1ppm of chlorine.

It's a lot to learn at first but you're doing great!