Visible algae, but with CC of 0 ~~ I'm confused


Aug 5, 2019
Orange County, CA
Hi all!
Seasonal pool owner here-- and no expert. Little history-- last year was my first year with this pool, and followed pool store advice. Year two: when setting up the pool, went to the pool store to discover it had closed! So TFP methods it is! This season has gone smoothly up until yesterday. I have the intex SWG that comes with the vinyl AG pool, but it isn't totally reliable-- I have to keep the salt levels at 1,000ppm for it to work (fought it all last year at "the correct level" with a near-constant error message of "high salt". So I consider the SWG as a supplement to LC; on days when we don't swim, it maintains the FC.

Anyway-- current challenge:
Two days ago, we went to swim to discover a "green dust" that swirled when the water was agitated. We don't have any trees over the pool, and we have a solar blanket that is on it when we aren't swimming, so I am assuming that it is algae. However, there is not even a tiny bit of sliminess on the pool walls or floor. (Last year, we experienced the slimy algae.) Also, my FC hasn't dropped below 7 in three weeks (we had a pool party 5 weeks ago that caused some numbers to fluctuate for about 2 weeks, but I've been getting CC of 0.5 or less consistently for three weeks).

Sat: before entering pool 4pm: FC 10 CC 0.5 pH 7.5
Entered pool: observed "green dust" and made sure to agitate all of it to increase FC access and to check for sliminess (of which there was none).
9:30 retest: FC 9.5 CC 1.0
Sunday afternoon: added some Miratic Acid and, an hour later, added 2 cups LC to keep from getting worse until I could tackle the SLAM)
FC: 13.5
CC: 0 (yes, zero. I can't figure this out-- water is cloudy and green)
pH: 7.3
TA: 90
CYA: 70
added 128 oz of 10% LC to raise it to 35 (overshooting the SLAM level of 28)
FC: 23
CC: 0

I can't figure this out-- I clearly have something in my pool that needs to be killed, but every time I keep getting a CC of zero. I've gotten CC levels of 1 or 0.5 on previous tests, so I know what a positive result should look like. The reagent is basically new (expires 9-20). I have R-0870, R-0871, and R-0003. The remainder of my tests are done from the hth 6-way test kit.

Anyway, added 5 cups of 10% LC and turned on my SWG to maintain levels overnight.
This morning:
FC: 29.5
CC: 0
The cover had been on all night-- how in the world could my CCs be zero?
No visible green in the pool-- nothing moves when agitating the water, and very lightly cloudy.

Two questions: what is growing in my pool? (research on TFP indicated possible mustard algae)
Why on God's green earth would my CCs be zero in a situation like this?!?

Thanks all for your help!


TFP Expert
Mar 2, 2011
Plants, such as algae, create chemicals like glucose from carbon dioxide and water. Using light, the carbon in carbon dioxide oxidizes the oxide into oxygen.

6CO2 + 6H2O --> C6H12O6 + 6O2

Carbon dioxide + water--> glucose + oxygen.

This is how plants make oxygen and store energy in sugars. Most of the bulk of plants is carbon from carbon dioxide. Most of the weight of a tree comes from the air.

The carbon in carbon dioxide is in the +4 state.

The carbon in glucose is 4 at 0, 1 at -1 and 1 at +1. The carbon is "reduced", which is the opposite of oxidized. Reduced just means that the oxidation state is lower or reduced because the atom gained electrons which are negatively charged.

When chlorine oxidizes the carbon in glucose back to a +4 oxidation state, it reverts back to carbon dioxide.

This is a similar process to an animal using oxygen to burn sugars to release energy and exhale carbon dioxide.

So, chlorine reacting with algae is mostly an oxidation reaction and not a combination. Algae is mostly converted back into carbon dioxide and water.

Chlorine can combine with carbon compounds, such as methane (CH4) by replacing the hydrogen ions.

Carbon in methane is in the -4 oxidation state.

Algae mostly does not create CCs.

CCs are mostly created by compounds like ammonia where the nitrogen is in the -3 state. For ammonia, you get combination and oxidation. So, the CCs eventually go away, especially in sunlight where UV photons knock loose electrons from the nitrogen and make it easier for the chlorine to take them.

Active chlorine is +1, so it bonds with more negatively charged atoms, like nitrogen in the -3 state or carbon in the -4 state.

The carbon in algae has a zero net charge.

So, the reaction of chlorine with algae is mostly oxidation and not combination.
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Aug 5, 2019
Orange County, CA
My SWG is still on to help maintain levels. Haven't added bleach yet today.

1:30pm (cover off to burn off CCs)
FC 28
CC 1.0

FC 27.5
CC 0.5 or less (it barely tinged pink, but cleared with one drop)

No sign of the "dust" on the bottom, and visually appears clear.

Is it possible I've cleared it in one day? OCLT still to be done, but I didn't think it would be ready to try by tonight...