Why is there algae in my neighbor's pool?

[bluenoise]

My neighbor is out of town on vacation and he's asked me to go over and toss a little bleach in the pool for him. Since he left yesterday, this morning was my first visit to his pool. There is a pretty solid band of algae from about 8" deep to about 24" deep. Seeing a large bucket of pucks poolside and three floaters in the water, I feared the worst regarding stabilizer. I grabbed a sample of water and brought it home to test. The results are puzzling to me.

FC: 6
CC: 0
Ph: 7.5
CYA: 40 (!)
Temperature: 84
I did not test TA or CH.

To me, the numbers look ideal for algae prevention, yet he's got quite a bloom going there. The only thing that seems off to me is that the pool equipment wasn't running when I went over. I don't know when he runs it or for how long, but inadequate circulation is the only thing I can think might be allowing that algae.

 

[JasonLion]

Keep in mind that the chlorine level required to kill algae is much higher than the chlorine level required to prevent algae from getting started. If you get algae and don't bring the pool up to shock level it is possible for there to be algae growing on the walls and chlorine in the water (particularly with three floaters).

This isn't a stable situation, the floaters are adding CYA and the algae is using up chlorine at a substantial pace. Chances are the algae will get ahead at some point, but even if it doesn't the CYA level will go up and make recovering much more difficult. The pool really should be shocked.

 

[Guest]

CYA: 40 (!)
If the pool is cloudy then this might actually be lower than 40 ppm.

inadequate circulation is the only thing I can think might be allowing that algae.

That is probably what is going on.

 

[bluenoise]

I called my neighbor to see if he had algae before he left. He said he's always had some, but doesn't want his chlorine up to shock level because they like to use the pool every day. Well, as long as he's fine with it being that way, I won't worry about it, but I did tell him he should take a few days off from swimming and shock it back to health so he doesn't keep wasting bleach trying to keep it chlorinated with algae consuming it all.

It turns out the bucket of tablets and the three floaters are empty. I don't know why they're all still there, but he switched to bleach some time ago after learning about the stabilization problem that comes with the tablets.