Will low CYA cause green algae?

8ightiesfan

Active member
May 26, 2020
36
Canberra, Australia
I have a small green algae outbreak so am performing a SLAM currently but would like to learn why it happened.

I haven't tested CYA in 3 months over winter (Australia) and it last tested at 70 but when tested today it is now 40. All other results were good

FC 8.5
CC 0.5 (first time it has been above 0)
PH 7.6
TA 80
CH 400
CYA 40


Would the low CYA have caused the outbreak?
Also can someone tell me if they can see my pool math logs in my profile please?

Cheers
Nick
 

aussieta

Gold Supporter
TFP Guide
low cya is not your problem
what is your water temperature
is your swg still working with cold water or are you using liquid chlorine over winter
cant check your logs as i dont use the app so they dont show for me, but nobodies do
my guess, water got too cold for swg and fc dropped
 

8ightiesfan

Active member
May 26, 2020
36
Canberra, Australia
Pool water has been warming up with solar heating and currently around 22-24c, 72f

The SWG was running as normal over winter, pool wasn't closed. FC levels were 6-7 over winter consistently.
 

duraleigh

Admin
Mod Squad
TFP Expert
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LifeTime Supporter
In The Industry
Apr 1, 2007
34,517
Sebring, Florida
Simply put, algae cannot grow in the presence of adequate chlorine.

Your chlorine has gotten too low or perhaps your circulation has allowed pockets of poorly chlorinated water to exist.

Inadequate brushing allows algae to form also as you develop areas where the chlorine cannot get to.
 

Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
8,624
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Sounds like you let one or more of the chemistry levels slip out of range, and/or maybe got a little, uh, lazy with the maintenance? I don't think it's any more mysterious than that. That's how I got a little of the green a few months ago. My FC dropped a half point below minimum for just a few days, that's all it took... Keep the FC and CYA within range for an SWG pool, and brush regularly. If the pool is open, you don't get to take a three-month vacation from that...
 

gregsfc

Bronze Supporter
May 27, 2014
204
Cookeville,TN
Based on your question, it doesn't seem as though you're understanding the relationship between CYA and FC thoroughly, which is probably the most important tenet of TFPC. Please read or re-read the "pool school" e-book. Click on this link; Pool School - Trouble Free Pool ; scroll to the bottom of the page; download the free e-book on a popular app like Google or Amazon; and read or re read the short e-book to get a full understanding of this all-important relationship.

If you've read through the pool school e-book and understand CYA's role in preventing algae, you'd know that a CYA level that this group considers too high (that is anything above 90) is such that keeping FC high enough to be effective in preventing algae would be impractical. For instance, if CYA were 200 in a manually-chlorinated pool and one extrapolated the math that is used to create the chlorine/cya chart, then one would need to target and dose FC all the way up to 17-19 ppm daily to ensure that the FC level never dropped below 15. Otherwise, the FC level would be too low to be effective at that CYA level, and if there were some sort of algae outbreak that occurred at this hypothetically-high CYA level for either an SWCG or manually-chlorinated pool, one would have to SLAM maintaining an FC level of 80, which would require massive amounts of liquid chlorine to reach and maintain that level. You won't see those kind of numbers in the TFP charts, because TFP recommends drain/refill above CYA 90, which is way below 200, and this is because anything above CYA 90 is an impractical way to manage a chlorinated pool.

A CYA level that is too low, in a way, is a different sort of problem. The proper CYA level (30-50 for manually-dosed pools and 70-80 for SWCGs) is the optimal ranges to protect accelerated FC loss to UV rays. So the problem with CYA that is "too low" is that any chlorine that you add to bring FC up to an acceptable level for preventing algae, whether it's added manually or via an SWCG, could be quickly lost during daylight hours and your pool could be sitting at or near FC 0 at any given time, and that's a situation you never want even for a second. So even though the required minimum FC level is lower for a pool that has a lower CYA level, you still must never drop below a minimum FC level, and that minimum level is always higher than zero regardless of the CYA level. But like it has already been answered, the CYA level by itself does not cause algae whether its too high or too low. It's has more to do with CYA's effect on how chlorine works or doesn't work in your pool to prevent algae.
 
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Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
8,624
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
What a lot of us here practice is to ignore the minimum FC number (for our CYA level) and use the low end of the target FC number/range as our minimum number. A sort of false bottom. We treat it like minimum, in that we never let FC drop below that number. Even at the end of our chlorine dosing cycle. That leaves a safety margin of FC in the pool at all times, which can cover the odd anomaly: a very hot day, a pool party, a dead animal trapped in the skimmer, whatever. If you play chicken with the actual minimum FC, then an unexpected FC-sucking event will drive you under, and that's all algae needs to get a foothold. Once it does, if you only continue to maintain the target FC, it'll get consumed very fast by the algae and you'll end up below minimum for a good part of your dosing cycle, and that will continue until you SLAM the pool.

I know you said you were maintaining FC all winter, but you might have let that dip once too often, or had some FC-using anomaly that drove it down quickly. Your CYA out of range exacerbated the event.
 

8ightiesfan

Active member
May 26, 2020
36
Canberra, Australia
Thanks for everyone's answers. I test FC, CC, PH every week and TA and CH every other week, just didn't test CYA untiI the pool would be used again. I don't believe the FC dropped below 6-7 at any point but what I do think is that with a CYA of only 40 with a SWG that 6-7 FC was not enough to stop an outbreak now that the water is warming up. The pool cleared up very quickly and passed an OCLT on the first night so we are back to normal again. Will now test CYA again and get it up to recommend level. I will definitely do some more reading on the FC/CYA relationship. Thanks!
 
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8ightiesfan

Active member
May 26, 2020
36
Canberra, Australia
Other thoughts that come to mind are, I tend to test in the morning so with a low CYA perhaps the FC is used up significantly during the day and replenished by morning so I never see the drop.

Also I recently had what may be considered a FC sucking event (see my other thread) I cleaned out the main drain to skimmer plumbing which I don't believe the previous owner had ever done. The was a basket full of organic matter pulled out which could have consumed a lot of FC perhaps.

I brush and vacuum weekly but one question I had been meaning to ask is should I also brush the floor after vacuuming? I usually brush the walls and vac the floor, rarely brushing the floor.

Cheers
 
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Dirk

Gold Supporter
TFP Guide
Nov 12, 2017
8,624
Central California
Pool Size
12300
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Sounds like it might have been a combination of things creating a perfect storm...
 
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