Stablizer off charts, phosphates high, ...

Hi - 10-12 yr old, 23000 gal above ground, vinyl liner, sand filter. Having problems keeping chlorine levels up, hence, bout with pink algea. Finally gone, but with all superchlorinating with dichlor, 6#, added more water and another 6#, still cannot keep chlorine up. Had water sample done, stablizer reading 300 and test for phosphorus 500. Suggested 50% water replacement, but would like to try to keep pool until fall and drain then. Sent me home to try 5# non-chlorine shock and qt of phosphorus remover as stop gap. Not so much worried about stablizer, but would like to be able to keep chlorine active for now. HELP!