With only 5500 gallons, my first thought was drain the pool and refill it but your fill water may also have very high alkalinity. If it does then you may want to follow Ben's proceedure for lower alkalinity:

http://www.poolforum.com/pf2/showthread.php?t=191

250 ppm is a long way to go so you will need to be patient. Also, check your total chlorine aganst your free chlorine, you may need to shock the pool.