NewB, IMHO your tub is not safe, because you've let FC drop to zero for too long. You have a VERY high Cholrine Demand, so that makes it even harder to keep FC up. I seriously recommend doing a Decon, preferably with Spa flush. However if you don't want to (can't) change your water twice you can just do this.
Add 1-2 Cups MPS, then 1 Gallon of bleach.
Let jets/air run for an hour, and let sit for a few more.
Wipe down spa (and cover) with super chlorinated water.
Check FC.
At this point I recommend refilling. However, if you don't want to refill yet, you can add Hydrogen Peroxide to lower your FC. It will take approx 1 cup to lower your FC by 10 ppm. I would lower FC to around 50 ppm if not already there, then let sit overnight. Check FC again to see what your CD is. If CD is 50% or below, you're good to go. Then do a refill ASAP.