Let me ask 2 stupid ?'s Why do I have algae?

[edweather]

Our pool in upstate NY was shocked when it was opened in April. Since then I have tested regularly with the TF-100 kit, and all the numbers are basically good. PH 7.5, TA 100, CYA a little high @ 60, and FC 11. The FC might seem a little high, but I've been battling a high CYA, it was as high as 90. I did let the FC get below the target level briefly a couple of weeks ago as an experiment, but not lately.

Today I noticed a little algae in several places in the pool. Why is this? I thought algae wouldn't grow if the FC level was ok. Should I shock the pool?

My next ? is, how much FC is safe to swim in? Let's say I shock the pool to 20ppm FC, is it ok to swim in? Thanks much in advance.

 

[Ohm_Boy]

Algae can get a good start when the FC drops below minimum levels. Once it's growing, you need to shock the pool to kill it. You should raise the FC to shock level and keep it there until you have no overnight FC loss, 0.5 or less CC, and your water is clear.

As for swimming, we generally accept that the chlorine level is technically safe up to shock level for a given CYA. Some folks have issues and would rather wait until it drops back to normal maintenance levels.

 

[edweather]

OK, thanks. Will do.

 

[kenmar]

How many hours a day do you run your pump/filter?
If your pump is not run for an extended period of time (even with high FC levels), localized areas where all the chlorine has been consumed can develop. These areas may then develop algae.

 

[waste]

Even with a sufficient level of chlorine in most of the pool, there can be areas of low 'interchange' which will allow algae to develop

The best thing to do in such a case is to brush or somehow manually increase the interchange - sorry, but it happens - a couple minutes every other day certainly beats an algae bloom!

 

[edweather]

Thanks again. I run the pump approx. 12 hours/day. I like the localized areas/low interchange theory. That might have been the problem. I shocked and brushed the pool yesterday. Things look good...will brush again today. I'll keep better track of my target FC level, and see what happens. If I still develope algae in a few areas, I'll just brush it more often.

 

[chem geek]

If there are areas of poor circulation, then as was noted the local chlorine level can get depleted, most especially if algae has already formed and taken hold on a pool surface (the same is true for bacteria forming biofilm, though bacteria are much easier to kill than algae if not in a biofilm). Regular brushing helps to detach any already formed algae, but maintaining a proper chlorine level at all times will help prevent the problem in the first place because algae do not spontaneously generate -- they must enter in the pool from the surface (or via a bather or other object going into the pool) and with proper chlorine levels they get killed before they can reproduce or migrate and attach to surfaces in areas of poor circulation. Prevention is key.

In your case, having the chlorine get too low for some period of time (as an experiment) probably had an area of the pool have very low chlorine levels and the algae then took hold.

 

[edweather]

I agree. I have to assume that letting my FC fall briefly below my target FC was the start of the algae. So now I'll be more vigilant and see what happens. Thanks.