Is this algae?

[TechJPC]

Glad to hear that you passed the OCLT.

Your CYA drop could have happened when you replaced water during the backwashing process.

I never backwashed or replaced water. I'll do CYA again tonight and tomorrow night also.

 

[kcindc]

You only need to test CYA if you replace a lot of water. The CYA test is very sensitive to lighting conditions. It is best to do this test outside in bright sunshine, with your back to the sun and holding the vial at waist level. You can pour the solution back and forth from the mixing bottle to the test vial to confirm your results.