Since you said it's free-form, I'm guessing you have a hard liner--concrete/gunite/shotcrete/plaster or tile. In which case, your pH won't hurt your pool. If it's vinyl, you need pH to be at a minimum 7.0.
When you added 7.5 gallons of 6%, you would have added 16 2/3ppm of free chlorine. If your CYA is 60ppm or higher, that won't be enough. Your FC dropped to 0. That tells me you probably have no CYA and lots of algae (algae WILL consume CYA after a couple of months of no cleanup).

So, follow WaterMom's advice.

FYI: for your 27,000 gallons, one gallon of 6% adds 2.2 ppm of chlorine. Figure accordingly. If you use 12.5% LC (Liquid Chlorine), each gallon will add 4.6 ppm of chlorine. Regular 5.25% bleach will add just under 2 ppm per gallon.