Algae came back unexpectedly

[ddas]

15k gallon in-ground pool, SoCal, with a SWG that had been malfunctioning, but I just put a new salt cell in there today so in theory it will be helping with chlorine from here on out.

I accidentally let the chlorine get to zero a month ago, and saw orange algae growing on the side of the pool. I treated it using the TFP recommendations, elevating the chlorine to shock levels via liquid chlorine, brushing multiple times daily, vacuuming, etc. Then I performed the OCLT and thought I had passed. So I put my solar cover back on the pool and assumed I'd let it drift back down to regular levels. The solar cover, in addition to warming the pool, also seems to preserve FC -- FC drifts down a lot slower with the solar cover on than without it.

1.5 weeks later (i.e. today) I took the solar cover off to do the weekly vacuuming, and I spotted more algae. What gives?

I checked the stats today:
FC: 10
PH: 7.0
TA: 90
CYA: 50

I have three ideas I'd like to run by the TFP gang:

1) Is it possible the solar cover had anything to do with it? Can the solar cover hold on to algae? If so, then I inadvertently cleaned the pool and then trapped algae in there. How do I get algae off the solar cover??? (It is VERY difficult to rinse due to its size and the size of my yard.) Or is it sufficient to re-shock the pool leaving the solar cover off? (Will algae continue to live on the rolled-up solar cover sitting on my driveway?)

2) My phosphates have always been high in this pool and they always drift upwards over time. It's been years since I treated them. I have some phosphate remover...should I just use that to remove more algae food from the pool? (Note: unfortunately the phosphate remover directions say to wait until FC is 1-4...which means I'd paradoxically have to leave the algae untreated for a few days until the FC goes down, THEN apply phosphate remover, then, I assume, shock the pool again.)

3) After the algae outbreak 1.5 weeks ago, I didn't clean the filter. Should I have? If so, how long after? I did look at the pressure gauge and it rose by a tiny amount after the algae cleanup, but nowhere near the 10 points that I normally wait before cleaning the filter.

Any thoughts on either of these two, or other things I should think about?

 

[needsajet]

A friend of mine is a pool tech who told me he has had algae problems in pools with covers, and almost never in pools without covers, suggesting risk is increased. I'm pulling up a chair on this one to see what the experts say. If it were me, I'd ignore phosphates and the filter, and re-SLAM.

Would depend on the limitations of your yard, but I'd probably lay it out on the driveway, pour some weak bleach on it and scrub it with a broom, multiple times, maybe take it to a park and lay it in the sun for a few hours. If I couldn't do that, I'd buy a new one or not use it for a while and see if the pool stays clean. Is it possible to sink a pool cover in the pool to SLAM it? Maybe some PVC pipes filled with sand and capped or some pavers or something like that to hold it down? I'd also be chasing down any other possible algae farms like under ladder steps, edge of coping, light niches, rails, back of skimmer weir, etc.

 

[tim5055]

My first thought is that it doesn't sound like you finished the SLAM Process

Did you pass all three criteria?

Yes, a solar cover can have algae growing on it.

We generally do not recommend phosphate removers.

I don't think the filter has anything to,do,with it. We recomend cleaning when pressure rises 20-25% over the clean pressure.

 

[Jezza]

+1 to needsajet's idea with the solar cover and need to recheck all the 'hidey' places for algae. You are going to need to SLAM the pool but I would just keep the cover off for the whole time.

 

[ddas]

(OP here.) Thanks a lot for the replies. I'm pretty sure I finished the SLAM method and OCLT properly the first time, but clearly, I need to do it again, since the algae recurred yesterday.

I'll try it without the pool cover (sniff, sniff, I have a pool party in two weeks...). I'll also keep the waterfall running this time (it runs into the pool), which will help the water circulate much faster. Out of curiosity, if it's a 15k gallon pool, and I SLAM it today and brush it a few times during the day, is it conceivable that I could be ready to run the OCLT within a couple of days, be home free, and be able to put the pool cover back on to prep for my pool party?

Water color is crystal clear, BTW, after I brushed and vacuumed yesterday. FC is still 10 but I'll raise it to ~25 or so to SLAM it one more time. Plus the new salt cell will be making chlorine for me now too.

 

[Richard320]

Two weeks is more than enough time if all you've got is some algae on the walls. People have cleared black sludge ponds in less time, with visibility down to a couple inches when they began.

Besides the high chlorine, don't neglect the brushing, and checking behind lights and beneath ladder rungs. Check the cover, too. It might have residual algae on the underside that recontaminated your clear pool.

Could you update your profile? SoCal covers a lot of ground... Barstow is different from Santa Barbara is different from San Diego is different from the San Fernando Valley.

 

[ddas]

(OP here.) Thanks Richard and everybody. I'm in the San Fernando Valley (Los Angeles). Dry and strong sun when summer finally hits us (clearly a little later than usual this year).

I've SLAMmed my pool now (FC=32 as of this morning), am leaving the waterfall running, am brushing the pool (twice today already and even though the water looks clear, when I brush I see the telltale orange-ish dust clouds swirl up slightly). I'll keep that up for the next couple of days, then reevaluate.

You mentioned "check the cover" -- I'm not really sure what you mean. I can look at it, but I don't see ANYTHING out of the ordinary. It's a bright blue solar cover (the usual, bubble-wrap kind), and I don't see anything on it. I'll leave it off for now, although, as stated previously, I'm anxious to get it back on. I wonder, if microscopic algae is on it, how long it would take to die, now that the cover is rolled up and out of the water.