Anna, if they test CYA @ zero, get three lbs from them and put it in your pool. This should bring you to about 30ppm and will help you keep Cl in the pool. If they test higher than zero, buy enough to bring your CYA up to 30-40ppm (you have 13,500 gallons in your pool).

Your gonna' need some more Cl. Buy some bleach while you are out and we'll help you on quantity when you post some test results.