Algae (or something) appears at FC of 10

[DivingIn]

I have a crystal clear pool and have been enjoying it this summer. I use the TFP method.

Last year while on vacation, the FC dropped to 0 and there were slight grayish hazy areas on the bottom, which I assume was algae. When I raised the FC to shock level, the areas disappeared. However, since then, the slight haze keeps coming back if the FC drops below 10 or 8. Raising it to 12 or higher causes it to disappear.

This week, I am SLAMing the pool (2 days so far). The FC drop today was around 8 (although it was a bright sunny day, and my CYA isn't very high). So when it dropped below 10 this afternoon, there was the slight grayish haze again. There is never any spots on any other part of the pool, and as I say, the water is crystal clear, the vinyl is 2 years old, and the ladders are clean.

If I am getting rid of algae by SLAMing, shouldn't the algae at least stay away at FC of 10? Could it possibly be something besides algae?

Right now:
FC: 14
TC: 14
CYA: 30
TA: 120
PH: 7.4
Temp: 82

I use FAS-DPD.

Thanks for your help.

 

[BuckeyeChris]

Have you done an overnight chlorine loss test?

 

[DivingIn]

Doing it tonight.

 

[DivingIn]

Overnight chlorine loss was 0. (18 to 18).

Yet from experience I know that when the FC goes down to 8 or so, a very slight gray haze will form in areas on the bottom of the pool.

 

[pabeader]

Sounds like a circulation problem, maybe. If you aren't getting adequate coverage of that part of the pool, the chlorine is only getting there due to density.

 

[DivingIn]

I've been using an automatic vacuum a lot in order to circulate completely. I also have taken water samples from the bottom with the same results.

 

[Texas Splash]

Since you stated you passed an OCLT, I wonder if "perhaps" your root problem is that with a relatively low CYA, perhaps what is happening is that the FC is being overwhelmed during the daylight hours (sun). You begin to see the clouding effects of the sun, then the sudden increase in FC (bleach) temporarily resolves the issue. Maybe you simply need to increase your CYA a bit to see if that provides the relief your FC needs during the day?

 

[DivingIn]

I may try that. I am reluctant to raise CYA because I know the problems of too high CYA. Just to be clear, the water never gets cloudy. There is a slight grayish color that develops on the bottom of the vinyl liner (both deep and shallow end). The slight darkening always appears in the same spots, and covers approximately 15-20% of the bottom.

 

[Texas Splash]

Normally I might also consider water circulation or filter issues, but the fact that it keeps happening only when your FC level drops keeps bringing me back to chemistry. I suppose process of elimination is in order. Ensure good water circulation to that 10-20% area, evaluate filter conditions, then go back to chemistry. Very odd though. I'll be interested to know what other ideas are provided, or if any of these suggestions helps.