Hi dubele,
I believe I was one of those that suggested you may have alage, even though your water is clear.
Algae is a single cell plant which is so small you can't see it without magnification and it multiplies quickly.
Not until is had multiplied to the extent that your water becomes cloudy or turns green do most people realize they have it.
Classic initial signs that you have algae are
1. Your pressure on the filter rises somewhat quickly
2. You cannot keep FC in the pool
To determine if your pool has orgainics/ algae
perform an overnight chlorine loss test so as to remove sunlight burn off from the FC loss equation
If you loose more than 1 ppm FC overnight, then there are organics in the pool which are consuming your FC