Last year we bought a used 24'x4' aboveground pool with a sand filter.(our 1st pool) I spent all summer reading this forum and we seemed to have everything going good. At the end of the season we let the chemicals get unbalanced and had some pretty bad algae. I added lots of bleach, some shock and then a bottle of that stuff that causes the dead algae to sink to the bottom. I vacuumed to waste and everthing seemed good. However, several hours after vacuming there appears to be some debris left in the middle. This has been going on all month. When I get close to it with the vacuum it appears to "puff" into thin air and disappear. Once again after the water is left undisturbed for several hrs, it reappears in the middle. Looking closely at the water it doesnt appear as clean and clear as last summer. I mean it looks like clear water but with tiny particles in it, as if its not filtering everything out. When I backwash the water in the clear bottle doesnt look very dirty.......Suggestions?