CL, CYA, and Algae

[TarheelFan]

I am confused. I thought I had this all figured out, but now I am unsure.

First off, here are my numbers:

FC - 2.5
CC - b/t 0 and 0.5
PH - 7.5
CH - 80
TA - 90
CYA - 40

Here is what has me confused. Last night I added a ton of CalHypo. I tested the water about 2 hours after adding it (a couple of hours before sunset, but late enough that my pool is completely shaded from the sun) and my FC was 6.0. Thus, I lost 3.5 FC overnight. That is alot more than my pool usually loses. Thus, I am concerned that I might have some unseen contaminants in the pool. One other thing...I have alot of stuff that looks like sand on the bottom of my pool. I thought it was undissolved chlorine so I swept it up to mix it, but now I wonder if it is dead algae or something else.

Here is what I am wondering:

Looking at the CYA / CL guidelines (both from this forum and the poolforum), the minimum CL for CYA 40 is 3. What exactly does that mean? Is water under 3 ppm FC going to allow algae to grow? Or is it simply not as effective but still effective enough to kill stuff?

I have not allowed my FC to get to 0.0 but it does drift into the 2 range frequently before I add Chlorine. Do I need to be ensuring I keep it over 3 at all times?

Thanks in advance.

 

[woodyp]

That's EXACTLY what under 3 ppm for that CYA level means---allows algae to grow. Bump up your target FC level to cover.

 

[Butterfly]

If you lost 3.5 FC overnight then I am sure you realize that [shock:3pwdaw57]you need to shock your pool.[/shock:3pwdaw57]

After you complete the shock process, your maintenance FC should be between 3 and 7, and never below 3.

 

[TarheelFan]

Thanks for the responses. I have been slowly increasing my CYA, so perhaps when I was at 2 FC over the past month or so I was OK b/c the CYA was lower, but now it is not so I have allowed stuff to grow. That's a bummer, but at least I know now.

Also, does dead algae look like sand? If not, what might that sand like stuff be?