Solar Cover causes algae?

JohnT

Admin
Mod Squad
TFP Expert
Apr 4, 2007
9,461
SW Indiana
#1
I've seen posts from people complaining about having algae problems after leaving the solar cover on for a week or so. I always figured it was probably just neglect, but I saw it happen over the weekend to a friend's pool and now I wonder.

Buddy from work called me and asked me to check his pool yesterday afternoon. He's overseas for 3 or 4 weeks, and his sister emailed him and said the pool was green when they rolled the cover up Saturday. He has a SWCG and was running the pump 10 hours a day. His numbers were about perfect although I couldn't check his salt, and nothing seemed unusual that might have caused it. The pool was just a little green, with no algae on any surfaces. I shocked the pool and set his pump to run continuously and told his sister to check it this afternoon and call me.

Got me to thinking that maybe it was the rainwater on top of the cover that had grown the algae. Actually, any water that gets caught on top of the cover would be subject to algae growth since it can't circulate and is exposed to plenty of sunlight to consume the chlorine in it.

I'm thinking if I go back that I'll rinse the cover off with pool water to both expose the surface to chlorine and to flush any algae into the pool where it can be dealt with.

Anybody have any thoughts?
 

frustratedpoolmom

TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
12,177
SWSuburban Chicago, IL
#2
I've used a solar cover for several years; never developed green water under it. After returning from a 9 or 10 day vacation, whatever water might be sitting on top has on occasion been green from no circulation or chlorine reaching it, but its such a small amount I haven't had a problem.
 

Hotrod30

Platinum Supporter
LifeTime Supporter
Dec 22, 2007
504
Central New York
#3
I've had it happen to me with the solar cover on for awhile. There is no easy way to take the cover off without dumping that algae mess into the pool. I think you did the right things in dealing with it. The only thing I would suggest you do is to check the pool yourself acouple of times a week. It doesn't sound like his sister would deal with it; and your buddy would be happier to have someone looking after his pool.
 

CarlD

Well-known member
Apr 8, 2007
104
#4
JohnT:

I've run into this myself as well--inevitably after the cover has been on for a VERY long time, like two weeks or more due to rain or vacation. I had a mild bloom several years ago when a rain system stalled over New Jersey for 10 weeks--the TV news started calling it "The Big Eddy".

I'm guessing that the cover creates some sort of anaerobic condition that's conducive to algae growth--I don't think algae likes a lot of oxygen. Drop-in steps are also denizens of algae growth--I sometimes stick Cal-Hypo tabs in there to try to fight it. (One year I tried salt bags as weights--not a great idea but there wasn't any algae as a result) Plus, with the pump on lo speed (which you usually want while on vacation) the water may not move as well under the cover.

The best things I've found to fight this are a liberal use of Polyquat 60% in advance, followed by a solid regimen of adding chlorine. I usually have a friend or relative dump in a jug of bleach every two days--but sometimes even that doesn't work--like last summer, when I came home to a minor bloom (Hammered it--it was gone in 24 hours).

The one fool-proof way I had to prevent it was to put in four floaters loaded with Tri-chlor tabs. Came home to an algae-free pool--but CYA had jumped from 30 to 60ppm. Worse, pH had dropped off the low end of the scale and took two or 3 boxes of Borax to get it right.
 

JasonLion

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LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#5
There are a whole variety of chemical reactions taking place at the surface of the water that are stopped by a solar cover. Also, the UV in sunlight interacts with the chemistry in the water and that is blocked by a solar cover. It doesn't seem to take very much time uncovered to keep everything in balance, an hour every couple of days, or several hours less frequently, seems to take care of it in most situations. A similar thing happens to indoor pools that are in a room which does not get enough ventilation.
 

CarlD

Well-known member
Apr 8, 2007
104
#6
JasonLion said:
There are a whole variety of chemical reactions taking place at the surface of the water that are stopped by a solar cover. Also, the UV in sunlight interacts with the chemistry in the water and that is blocked by a solar cover. It doesn't seem to take very much time uncovered to keep everything in balance, an hour every couple of days, or several hours less frequently, seems to take care of it in most situations. A similar thing happens to indoor pools that are in a room which does not get enough ventilation.
Yeah, that's totally consistent with my experience, too.
 

CRG_80cc

LifeTime Supporter
Jul 9, 2007
81
Nor-Cal
#7
I have had this happen to me the 2 times we’ve used our solar cover so it now sits in the corner. From what I read here, it does seem to be because we left the cover on for over a week without letting the water “breathe”. I was vigilant about monitoring the water and keeping chlorine levels up, so I thought I was fine until we took the cover off.

I am thinking of experimenting by just covering ¾ of the pool which will allow the pool to breathe and still slow down water and heat loss as compared to it fully uncovered.
 

JohnT

Admin
Mod Squad
TFP Expert
Apr 4, 2007
9,461
SW Indiana
#8
CRG_80cc said:
I am thinking of experimenting by just covering ¾ of the pool which will allow the pool to breathe and still slow down water and heat loss as compared to it fully uncovered.
That kind of goes where my next thought is: Could you put some softball-size holes in the cover to eliminate the problem without ruining it for retaining heat? No clue what kind of area you'd have to open.
 

CRG_80cc

LifeTime Supporter
Jul 9, 2007
81
Nor-Cal
#9
JohnT said:
That kind of goes where my next thought is: Could you put some softball-size holes in the cover to eliminate the problem without ruining it for retaining heat? No clue what kind of area you'd have to open.
My solar cover is in 2 pieces, so I would just add the larger of the 2 pieces to get the 3/4 area coverage. The only problem I see if you put holes in the cover is that it makes it much harder to sweep any debris off of the cover.
 

CRG_80cc

LifeTime Supporter
Jul 9, 2007
81
Nor-Cal
#10
Just an update.

So I placed one of the 2 pieces of my solar cover back into the pool. So it's been a week and have been monitoring chlorine levels as I did without the cover. So far, no algae. Chlorine loss is less as expected. I'm adding about 1/2 gallon of 10% liquid chlorine every other day to keep it about 3-5ppm.

Last time I had it on, I had algae growth after having it on for a week even though I was adding chlorine. That was without ever taking the cover off. So it seems that letting the water "breathe" helps keep algae from growing at least in my pool.
 

launboy

Well-known member
Jun 15, 2008
582
S.E. Wisconsin
#11
FWIW, we leave our cover on all the time when not in the pool. Our average water temp. is about 90, and many times the chlorine level will drop down to 0. I am amazed we've only had algae once this season actually, and it was just a little on the floors. In the beginning of the season when it was too cold to swim, the cover would stay on for over a week at a time. We've never had any problems. Our pool water is changed over 3 times a day, because the filter runs for 9 hours to heat the pool with solar, so we have good circulation.

HTH,
Adam
 

CarlD

Well-known member
Apr 8, 2007
104
#12
Hmmmmm....I'm wondering if the covers are becoming contaminated sitting off the pool, wet. Usually, if I'm really on top of things, the cover can be on for days at a time and all I have to do is vacuum after it comes off.