So a pool cover does cause algae after all..

mart242

LifeTime Supporter
May 3, 2010
153
Ottawa, ON, Canada
The weather was chilly two weeks ago and last week the kids were sick so we did not get a chance to use the pool. I though that my trusty salt system would protect me from algae as it did all summer long but oh what surprise did I get Saturday afternoon when it was quite hot outside and the family got ready for a swim. A green pool!!

I vacuumed, backwashed and shocked the pool (still shocking) and it's getting better but I'm surprised that I got algae. FC was at 5.5 for the last month or so and CYA at 75. Shouldn't that be enough to protect the pool from algae even if the cover is on? I know that leaving covers on for a long time is a big no on regular chlorine pools but remember reading somewhere that it was a non issue with the salt pools.. I guess I was wrong.

Is there a guideline for the max amount of time the cover can stay on? Can I put the cover on and leave it for a week once my FC is at shock levels but I'm not losing FC? It will be quite cold this week so I doubt we'll use the pool much.

Thanks
 

PaulR

LifeTime Supporter
Jan 11, 2009
1,966
Cupertino, CA
People say CC can build up if you don't take the cover off for a while say, every week or so. I shouldn't think that reasoning would be different for bleach pools versus SWG pools.

That's different from "causing" algae, though. I've observed that I have to keep my FC a point or two higher than what The Chart says, to keep the algae away. I'm beginning to wonder if I'm doing something funky with my CYA testing... but it may simply be the case that some pools need a higher target/minimum than what The Chart says.
--paulr
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
Was the FC still 5.5 when you opened up the cover and found the pool green? It's possible that the temperature of the water in the pool rose when the weather got hot and the SWG couldn't keep up with the faster chlorine loss at higher temperature and dropped to allow algae to grow. If you set the SWG on-time lower when you covered the pool when the weather was cool then it could have been set appropriately for cooler water, but not for warmer water. How often did you check your FC level? With an SWG, one might not check as frequently and normally that's OK if chlorine usage is fairly consistent, but water temperature can significantly change chlorine demand (especially when there isn't sunlight since proportionately temperature becomes a bigger factor).

The only time I got algae was on a spring opening one year when the water was warming up and the chlorine usage went up quickly and I didn't catch it so it went to zero. The chlorine usage from the cooler water was very low and the increase in usage when the water warmed up caught me off guard.
 

4JawChuck

Well-known member
Jun 13, 2010
223
Winnipeg, Manitoba, Canada
Algae probably started on top the cover where the conditions are moist and warm and there is next to zero chlorine to kill it, add a little dew or light rain and you got algaeville! I submerge my cover once a week at least to keep it clean and kill off anything growing on it after I hose it off, I have noticed my chlorine demand go up after I do this so its eating something.

Lots of junk in the wind and all it needs is a warm and moist area to grow, next time submerge the cover to get the topside wet with chlorinated water before you leave it for a couple of days. In the mean time you should "shock" the cover while your doing the pool and get it clean with a brush, I would let it soak for a week while the pool is in shock and brush it everyday when you push it under to get the topside wet with fresh chlorinated water. I bought a carwash brush strictly for this purpose, works great on the slide and diving board too.

Good luck getting her cleaned up.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
That's a good point -- the cover itself can have algae and other organic debris that creates a higher chlorine demand, at least initially. If you had gotten algae within a week of putting on the cover then the dirty cover could have been a culprit. Since it took longer than that in your case (if I read your post correctly), it's not as clear if that was the cause but 4JawChuck's advice is good regardless since you don't want to introduce things that consume chlorine into your pool in general.
 

mart242

LifeTime Supporter
May 3, 2010
153
Ottawa, ON, Canada
I had set the SWG to 25% early august, it needed to be higher during july since the cover was off most of the time. Temperature was cold in the last two weeks so warm water is definitely not the reason why algae appeared. FC was at 5 when I took a sample of the water.. wouldn't that mean that no algae should be present? I guess the cover theory could explain that, ie: "stuff" developed on top of the cover and with a good rain it all ended up in the pool.

The water had a slight greenish tint but all the algae was on the pool floor and lower walls. When I vacuumed it, my sand filter couldn't filter everything and made the water a bit more greenish. I thought about vacuuming to waste but how can you do that when it takes a good hour to vacuum the pool?

The water was pretty much clear this morning, FC was at 29 but I poured another jug of bleach to raise it by ~4ppm since I'm at work during the day. I'm hoping that it will still be at shock level after work so I can put the cover and brush it in the high FC water. A good brush and vacuum tomorrow and then I'll put the cover back on for the rest of the week because it will drop down to a chilly 50 - 60F for the next few days..

I was super ****** on Saturday, the family god ready for a nice relaxing swim since it was a nice day and I removed the cover to find the green soup.. on one of the last few days of the season!
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
5 ppm FC with 75 ppm CYA and having green algae may have us rethink the lower SWG recommendations if this happens too frequently with others as well. If the algae was growing on the cover and got dumped in from rain, then it could have been in the process of slowly getting killed, but if it was growing in the pool and not just dumped in, then the non-SWG level closer to 6 ppm might be needed.

Thanks for reporting this and sorry it ruined your swim day. The chlorine/CYA charts seem to reduce the occurrence of algae to less than 1 in 1000 (perhaps even 1 in 5000) but being the "1" makes that statistic not very helpful. In your particular situation, you might consider unusual extraordinary measures. If the cover cleaning that 4JawChuck suggests isn't enough (try that first, of course), then either having a higher FC target especially when the cover is used or using a supplemental algaecide (unfortunately at extra cost) is an option.

Do you have borates in your pool? That could also potentially help as they are a mild algaecide.
 

mart242

LifeTime Supporter
May 3, 2010
153
Ottawa, ON, Canada
I have not noticed any green algae on the pool cover but it's not because you don't see it that it's not there.. then again, if you can't see it, you'd think that chlorine would be enough to kill it. The only visible thing was a dead frog / toad.. well bloated but I doubt that it was the source of the algae. There are no borates in my pool or algaecide. I'll add algaecide when closing the pool for winter though, it comes in the closing kit.

I tested for CC 2 or 3 hours after discovering the mess (once I was done vacuuming) and it was at ~1ppm. Does CC drop rapidly?

On another note: my pool now smells like a public pool. ;)
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
The chlorine combining with organics in the algae can produce CC and that can definitely smell. It's most likely monochloramine and possibly dichloramine that you are smelling and yes, that is associated with "pool smell" in some commercial/public pools. Chlorine will get rid of it, but you can make it go away faster by 1) removing debris and organics, most likely by getting the (dead, hopefully) algae to the filter and backwashing/cleaning (or by vacuuming to waste, but as you noted that may not be practical if the algae is spread around over a large area) and 2) shocking the pool removing the debris and organics. Exposing the pool to sunlight will also help accelerate the oxidation of CC.
 

offgrid

Well-known member
Aug 17, 2010
87
Ottawa, Canada
4JawChuck said:
Algae probably started on top the cover where the conditions are moist and warm and there is next to zero chlorine to kill it, add a little dew or light rain and you got algaeville! I submerge my cover once a week at least to keep it clean and kill off anything growing on it after I hose it off, I have noticed my chlorine demand go up after I do this so its eating something.
.
This works - I noticed a smell the last time I left my solar blanket on for a few days in the rain. I figured it was algae forming in the puddles of rain water on the cover. I also noticed small pink dot clusters on the edges of the pool (probably from the puddles that ended up in the pool)

So when I put the cover back on I followed your advice and submerged the cover so that it had a good amount of salt-chlorinated water on the surface... after a few days of rain here there is no smell this time.

Thanks 4Jaw.
 

4JawChuck

Well-known member
Jun 13, 2010
223
Winnipeg, Manitoba, Canada
My pleasure, I had problems with algae and couldn't figure out where it was coming from and then realized the cover, slide and diving board all had little black spots which could be sources...so I scrubbed and left all my pool equipment in the pool while I shocked for a week and the problem disappeared. I get a lot of debris on my cover carried by the wind so I spray it off with the hose towards the skimmer and then scrub with the brush to get it clean (trick I learned here at TFP!), I think a lot of my problem was caused by the cover being wet and then rolled up all day while we used it so I try to hose it off on a regular basis just to get the dirt off before we roll it up.

So far no problems doing it this way, I think the small amount of chlorine in the potable water from the hose is enough to keep the cover sanitized but I still dunk it every couple of weeks to make sure it stays clean...pretty amazing how much stuff gets carried in the wind. Must be at least a handful every night...mostly airborne dirt out here in the prairies from the farmers fields. :wink:
 

learthur

Gold Supporter
LifeTime Supporter
Sep 9, 2008
243
The Woodlands, Texas, USA
I also had algae episode using cover. was logging data at the time using bleach and the cover increased my chlorine demand by at least 3x.

Even had an episode last winter with the salt water pool.

Now I just run the chlorine at 150% of whAt the cl/cya chart indicates and no more algae.