The purpose of the overnight FC test (OCLT) is to determine if you have living algae, or other organic contaminates, in your water. If you do, you need to SLAM, or continue SLAMing, the pool. This test is most useful to confirm that the SLAMing process is complete, and can also be useful if you are unsure if there is a problem or not.

Mustard algae is a particularly persistent kind of algae, that appears to go away when you SLAM the pool normally, only to reappear as soon as you come back down to normal chlorine levels. It tends to grow in clumps on surfaces at normal chlorine levels, and varies from a true “mustard” yellow to green with hints of yellow. It can also grow free floating in the water if the chlorine level is low enough.

Black algae can be the bane of pool ownership. The spores enter swimming pools through the air and can take root in the plaster of an inadequately chlorinated pool. These roots, along with the waxy protective barrier it forms around itself, make it particularly difficult to remove. When it comes to black algae, an ounce of prevention is worth two tons of cure. Anyone who has ever dealt with a black algae infestation can attest to this. It is far better to avoid black algae by focusing your efforts on maintaining the proper chlorine (FC) …

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