I hit it with 0.5 gallons of bleach which, according to an online calculator, is 5 ppm and I'll treat any more trailer loads I add at 1 ppm. I've read online I need to keep it between 0.5-1 ppm.
I am also going to order test strips. Well pump is currently pumping into the tank (it times out and waits about an hour), so I'll have to monitor this thing weekly I bet.