gunnite pool with persistent algae

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etbrown4

Well-known member
Feb 24, 2012
75
We have an 8 year old gunnite pool with a dark gray speckled mini pebble plaster finish.

To the touch the finish is perfectly smooth and free of cracks or chips, and bonded tightly.

My pool guy uses tri-chlor or lots of the 3" tabs in the skimmers and he says he keeps the chlorine level at around 5-10 ppm, we still have persistent algae forming on the bottom and occasionally on the sides every 4-5 days. I have not tested the ppm to be sure he is correct.

Pool guy swears there is a problem with my plaster and he says it is slightly porous and allows the algae to hide and grow in the pours. He takes care of about 20 pools and says mine is the only one with persistent algae. He's been doing pool care for a long time.

I have seen him use the wire brush along the bottom and he can create a grey cloud apparently coming from the plaster while he is brushing, and that worries me some.

Anybody had experience with this kind of pebble finish and had a recurring algae problem?

Suggestions welcome.
 
Your CYA is likely quite elevated from using the 3" Pucks. This means your chlorine target is likely closer to 13-15. Unless your pool guy is taking the time to do an FAS-DPD test, he probably knows it is higher than 5, but not more than that.

This means your pool is under chlorinated, and should be taken through the shock process outlined here in Pool School. You really need a full set of test results, otherwise you will not really know the proper shock level, as well as how to get started getting other numbers in order. Reducing your CYA is going to require water replacement.

The gray dust observed while brushing could be dead algae, or plaster dust. If pH, TA and CH are way out of line, you could be destroying your plaster over time.
 
As Elwood eluded to, Your CYA could be very high and the rest of your chems could be out of range too. If you have a filter that you don't backwash (i.e. cartridge) that will cause your CYA to build up very rapidly. Also using a lot of tablets will lower your pH and low pH will attack the pool surface.

I suggest you get your own good test kit and start taking control of your pool.

Also, list all your pool and equipment.
 
I say it's time to fire your pool guy, he is either being lazy or doesn't know what he's doing, or most likely both. There are good ones out there, but in my opinion yours is not. A plaster problem will not cause algae. My plaster is in horrible shape, and I have zero algae. I haven't been able to replaster due to water restrictions and other problems with the house sucking up all my money. :(

Anyway, it's time to take control of the pool yourself. We can help!
 
etbrown4 said:
Hi All,

I have a Hayward Superpump and a sand filter. Yes he back washes all the time.
Click to expand...

Without a comprehensive set of test results, all of us will be shooting in the dark as to what is causing your problems. If Pucks have been used all along, CYA may still be very high, and your pool is therefore under chlorinated. Algae can be a chronic problem if that is the case.

I also have a Sand Filter, but even 3 minute backwashes, with another minute Rinse, every few weeks amounts to a very small percentage of the water.
 
If your FC and CYA readings are accurate, then your current FC is sufficient to prevent algae. However, if you have algae, then the CYA is likely higher than 100 ppm and/or your FC has been below 7.5 ppm.

Pretty much, as long as you follow the FC/CYA chart, you won't get algae.

At this point, I would suggest that you get the pH and TA up, and the CYA down to at least below 80 ppm, and preferably below 60 ppm.
 
What James said above is correct (as are the others). But I wanted to add that in your case right now, simply keeping FC at or above 15ppm won't clear your algae. It will maintain the water only after the algae has been eradicated by shocking the pool.

It will take a lot of chlorine to shock your pool at your CYA level. Shocking the pool at 100ppm is impractical and expensive. It's entirely possible it's well over 100ppm as well. Past 100ppm, the CYA test is far more inaccurate. Whatever the real CYA level is, it's at least 100ppm which means you need to drain at least 50% to bring CYA down to a manageable level. 50ppm however is still on the high side.

Once you get the CYA down you can shock the pool and eliminate the algae. If you can get the CYA level closer to 40ppm it'll take less chlorine to shock the pool so keep that in mind.
 

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I have some updated readings and some more info from my pool guy as to types of chlorine used.

CYA 125
H 200
TC and FC 15
PH 7.0
A 40

My Pool man continues to insist that with the porosity of my pebble plaster finish that algae inherently grows in the pores, unlike maybe some other gunnite pools with possibly less porous plaster or certainly fiberglass pools etc. I know a number of posters have said they have zero problems with dark grey gunnite pools!

The pool has a very high swimmer load and I've seen as many as 20 adults in the pool at one time. On these hot summer days it will go green in a heart beat unless he keeps the Chlorine in the 15-20 range. The average daily high temps have been 90-100 for weeks.

He says he has only had success in using many pucks in the skimmers and some shock twice a week. He believes the ratio of the pucks to the shock is about 4-1 or 4 times as much chlorine introduced with the pucks. He tries to keep it at 15-20ppm. I know this is not standard and not typical, and not desirable under usual conditions.

Two previous pool guys I've had in earlier years could not keep the algae out of the pool at typical 1-3 ppm levels. My current pool guy is just doing what he feels he has to do to make it work. I suspect there is a better way. Additional ideas are most welcome.
 
etbrown4 said:
I have some updated readings and some more info from my pool guy as to types of chlorine used.

CYA 125
H 200
TC and FC 15
PH 7.0
A 40

My Pool man continues to insist that with the porosity of my pebble plaster finish that algae inherently grows in the pores, unlike maybe some other gunnite pools with possibly less porous plaster or certainly fiberglass pools etc. I know a number of posters have said they have zero problems with dark grey gunnite pools!

The pool has a very high swimmer load and I've seen as many as 20 adults in the pool at one time. On these hot summer days it will go green in a heart beat unless he keeps the Chlorine in the 15-20 range. The average daily high temps have been 90-100 for weeks.

He says he has only had success in using many pucks in the skimmers and some shock twice a week. He believes the ratio of the pucks to the shock is about 4-1 or 4 times as much chlorine introduced with the pucks. He tries to keep it at 15-20ppm. I know this is not standard and not typical, and not desirable under usual conditions.

Two previous pool guys I've had in earlier years could not keep the algae out of the pool at typical 1-3 ppm levels. My current pool guy is just doing what he feels he has to do to make it work. I suspect there is a better way. Additional ideas are most welcome.
Click to expand...
Order up a proper test kit and do it yourself. My recommendation is a TF100 with the XL option. http://tftestkits.net. For starters, that pH reading is suspect with FC that high.

You'll want to drain off a good bit of the water to lower CYA. Then perform the shock process. Once you get it licked, as long as you maintain the FC at the proper level for your CYA level, you'll never need to do the the shock process again.
 
Richard's advice above is good. As he said, drain/refill to get down to 50 ppm cya, then doing the shocking process is what we would recommend, and will change your pool experience (i.e. no green showing up once a week and being able to handle high bather loads). Ignore the "rough plaster finish" excuse the pool man is giving you.

etbrown4 said:
I suspect there is a better way. Additional ideas are most welcome.
Click to expand...
There is a much better way, unfortunately your pool man doesn't have a clue about it. The pucks are his (and your) undoing. By continuing to add more pucks, the pool man is also raising your cya level, which means he also has to raise the FC level (which in his defense, at least he knows to go to a higher FC). This is an un-winable battle. See the Chlorine/CYA chart. By the way, it is likely your cya is higher than you posted number since most cya tests can not really measure above 100 ppm without doing some manipulation.
 
I've checked a few sites for cya testing at over 100ppm, as many posters expect that kind of level.

I've seen name brand testers at $17 for up to 100ppm, but really nothing at over 100.

Any ideas on how to ascertain and read levels above that with something other than strips would be welcome. (hopefully without spending lots)

Without knowing that I have 150 or even 300ppm, it makes it hard to make the case. I'd rather be sure. :)
 
We have a gunite pool that my husband used pucks and powder shock in for several years. We kept having to add more and more pucks and we were still fighting algae. Only once we tested with the tf-100 kit did we realize how bad our pool chemistry was. Our cya was immeasurable, even with the 50/50 dilution method. We finally dove in and drained 80% of our water, refilled, retested with cya of 40, used pool calculator to add a few things to get ph balanced and calcium up to save our plaster, and have truly had a trouble free pool since. I only empty skimmers, give a quick brushing, and had a bit of bleach daily. If I need to miss a day, I just add more bleach to bump up the FCC level to account for a couple of days instead. 7 min max each day. Go for it! They really can't help you get sanity back if your cya is high. Our plaster was being collected by our robot daily as well. It is quite expensive to get replastered.
 
See this pool school article for test kits:http://www.troublefreepool.com/pool-school/pool_test_kit_comparison You can just buy the just the cya test from tftestkits.net, but I would get the tf-100 that includes everything you need. It the the best bang for buck and has fast shipping.

You will have to do a dilution...50/50 dilution and you multiply by 2, 33/67 dilution and you multiply by 3 and can "roughly test" up to 300 ppm.
 
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