I don't know what happened. For 10 years, using Chlorine, I had no problems. Last year, I had a bad algea problem, and switched to SW at the end of the season. This year, I have not been able to get my Chlorine right yet. Someone suggested my Phosphates might be high, and depleting it. I tested and am at over 1,000 ppb. I ordered a Phosphate eliminator, which hasn't arrived yet. I plan to add that 1st. After my Phosphates are eliminated, then I will take your advice and shock, then stabilize.