After completing an AA treatment, adding metal sequestrant, bringing chlorine up to correct level and balancing
Is there a test to check the amount of sequestrant in the water or should it just be used periodicaly
There are usually subtle signs of staining or greenish tint to the water. Plastic parts, like skimmers, usually turn a little brown first. When you start to notice that an early dose of sequestrant will usually lift the stains so you don't have to keep doing AA treatments.
Taylor has a test kit for various phosphonate based sequestrants. The multiplication factor (ppm's per drop) changes based on the type of sequestrant (HEDP, ATMP, etc). But the kits are expensive, well over $100.