It can take chlorine awhile to react with some of the types of 'goo' found in pools. That's one reason for POP -- you need to wait awhile to see if you are *really* done, or of the 'goo' is just 'playing possum'!

Also, it's often true that the are big piles or layers of gunk on the pool walls or floor. Often, people get the pool almost all cleaned up (so they think) and then brush -- clear pools can be turned green instantly that way.

Bottom line: till your pool has been brushed and vacuumed AND held stable chlorine levels overnight for at least 2 days, you can't assume your done.

Ben