You can keep the pool stable by using bleach until you get your unit up and running. For reference to help you figure out how much to add, in a 28K gallon pool, each 121-oz. jug of Walmart's generic 8.25% bleach will add about 2.8ppm of chlorine to the pool. I don't know what your CYA reading is but if you look at the chart at this link, it will tell you what range you need to keep your chlorine in based on that reading and you can dose with bleach accordingly.

http://pool9.net/cl-cya/