I enountered the same problem 1-2 months ago.... found out the reason in the end. Reason? Calcium Precipitate. My water was cloudy even after 2 days in a powerhead.. same as you. Try doing a test on the water.. if your calcium and kh is low.. confirm its because of calcium precipitate.... The salt can no longer be used.. unless you don't mind low calcium and kh in yr main tank...