Is it normal for algae on sides of pool every 2 days?

[birdslikemypool]

hi everyone,

this is my first time taking care of a pool. i am in phoenix, arizona. it is over 100 degrees very often in the summer. my pool is in a very sunny area.

the pool water looks great, but i have to brush the sides of the pool like every 2 days because green algae forms. after i brush it, it looks great..but then 2 days later it returns again.

the filter is a DE filter. i've been setting it to run all day when the sun is out. 5am to 8pm.

i think the pool is 12,000 gallons. it doesn't have a diving board and there's no very deep end. it doesn't have a liner. instead it has little bumpy rocks..i think called "pebbletech" or something.

i have a robot vacuum thing that goes around and cleans the bottom of the pool. i never see algae on the bottom of the pool.

i have 2 floating things that hold chlorine tablets. every 2-3 days i check these and fill them both up. so there's probably 8-10 tablets total in them.

i have a test kit that gives these results:
total hardness: 1000
total chlorine: 1
free chlorine: 3
ph: 7.6
total alkalinity: 150
stabilizer: 150

can someone tell me why even though i keep free chlorine above 3ppm, i still have to brush off algae every 2 days? what's wrong and what can i do?

i was thinking of getting a 3rd floating thing to put chlorine tablets in. is that a good idea or is something else the problem?

thank you

[mas985]

First Total Chlorine cannot be less than Free Chlorine so I assume those are swapped (TC = FC + CC). So the combined chlorine is very high and the pool needs to be shocked. But because the CYA is soo high (150 ppm), you will need a lot of chlorine for an effective shock. A CYA of 150 ppm means that your normal chlorine level should be 15 ppm (10% of CYA) to prevent algae and 60 ppm (40% of CYA)) to shock the pool!. You might be better off getting your CYA down first so you don't need so much chlorine to shock and for maintenance. This will require at least a 50% refill. Once the CYA is down to a reasonable level, then you can more easily shock the pool and maintain levels of FC.

[chem geek]

Mark (mas985) is correct. The reason you have algae and have a hard time maintaining a higher Free Chlorine (FC) level is that your Cyanuric Acid (CYA) is too high. The amount of active chlorine (hypochorous acid) that kills pathogens, prevents algae growth, and oxidizes bather waste is proportional to the FC/CYA ratio. So as the CYA level climbs, you need to raise the FC level to compensate.

You are using Trichlor tabs/pucks. This is stabilized chlorine and will increase the CYA level over time, The following are chemical rules of fact that are independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

So even with a low 1 ppm FC per day chlorine usage, after 6 months of Trichlor use the CYA would increase by over 100 ppm if there is no water dilution. Please read the Pool School for more information on how you can better maintain your pool.

[birdslikemypool]

thanks for the help. i just moved into this house about a month ago, so i inherited this pool condition. trying to correct it now.

if i replace some of the water and get the CYA level lower, how do i prevent the CYA level from getting too high again?

chem geek says just 6 months of Trichlor use could get the CYA level high again.

will i have to routinely replace some of the water every 6 months? what else can i do to prevent this from happening again after i fix it?

thanks

[riverdogg8899]

You may have a couple things going.
1. clean your filter. Don't just backwash. Clean 100%
2. you may be getting a false stabilizer reading from tabs. Most of your readings look good. Try addeing liquid.
3. use a metal brush to brush pool. Not just nylon.
If you need help there is a lady who knows her pools really well.
Give her a call. She may be able to help you.

Good luck,

Rob

hi everyone,

this is my first time taking care of a pool. i am in phoenix, arizona. it is over 100 degrees very often in the summer. my pool is in a very sunny area.

the pool water looks great, but i have to brush the sides of the pool like every 2 days because green algae forms. after i brush it, it looks great..but then 2 days later it returns again.

the filter is a DE filter. i've been setting it to run all day when the sun is out. 5am to 8pm.

i think the pool is 12,000 gallons. it doesn't have a diving board and there's no very deep end. it doesn't have a liner. instead it has little bumpy rocks..i think called "pebbletech" or something.

i have a robot vacuum thing that goes around and cleans the bottom of the pool. i never see algae on the bottom of the pool.

i have 2 floating things that hold chlorine tablets. every 2-3 days i check these and fill them both up. so there's probably 8-10 tablets total in them.

i have a test kit that gives these results:
total hardness: 1000
total chlorine: 1
free chlorine: 3
ph: 7.6
total alkalinity: 150
stabilizer: 150

can someone tell me why even though i keep free chlorine above 3ppm, i still have to brush off algae every 2 days? what's wrong and what can i do?

i was thinking of getting a 3rd floating thing to put chlorine tablets in. is that a good idea or is something else the problem?

thank you

[chem geek]

thanks for the help. i just moved into this house about a month ago, so i inherited this pool condition. trying to correct it now.

if i replace some of the water and get the CYA level lower, how do i prevent the CYA level from getting too high again?

chem geek says just 6 months of Trichlor use could get the CYA level high again.

will i have to routinely replace some of the water every 6 months? what else can i do to prevent this from happening again after i fix it?

thanks

You have several options.

1) You can stop using Trichlor and use chorinating liquid or bleach instead. This prevents the buildup of CYA (or of CH from Cal-Hypo), but requires daily or every-other-day chlorine addition unless you have a pool cover that is mostly opaque in which case you could add chlorine around twice a week (this is what I do in my own pool and what tens of thousands of pool owners at multiple pool forums do) or you can have an automatic dosing system (e.g. The Liquidator or a peristaltic pump). If you have water dilution that drops the CYA level slowly, you can use Trichlor every once in a while to fill in or when you go on vacation.

2) You can continue to use only Trichlor tabs/pucks and ignore the CYA level, but use a supplemental algaecide to prevent algae growth. The most effective would be copper ions, BUT that has a risk of staining (especially plaster surfaces) and having blond or gray hair get a greenish cast so isn't normally recommended unless you were to carefully test your copper ion concentration and keep the pH lower. Other algaecides, such as PolyQuat 60, would probably let you have the CYA get to 200 ppm or so (with 3 ppm FC) before algae would start to grow faster than chlorine can kill it (this first shows up as an increase in daily chlorine demand, then as dull/cloudy water, before a full-fledged algae bloom). You could use a phosphate remover, but that will be like PolyQuat in helping somewhat but not completely, especially if there are organic phosphates in the water (phosphate removers only remove inorganic orthophosphate). Note that preventing algae growth may not be enough since the higher CYA levels mean slower oxidation rates so if you have anything but low bather and organic loads, the pool may get cloudy more quickly, especially if the pool is not exposed to sunlight.

3) You can continue to use Trichlor pucks/tabs, but shock weekly with chlorinating liquid or bleach to kill off nascent algae growth. This will only help for a while and eventually the CYA will get too high for this to work, but it will buy you time. This is also similar to doing a partial option #1 -- that is, alternating between using Trichlor and using chlorinating liquid or bleach or supplementing between the two. Many people using Trichlor pucks/tabs do this hybrid approach because that's what most pool stores tell them to do. However, if you properly maintain your pool with the appropriate FC/CYA ratio, you never need to regularly shock your pool. The weekly shocking when using Trichlor just makes up for the active chlorine level being too low due to the CYA building up.

4) You can raise the FC level proportionately as the CYA level rises. You would need to do this using chlorinating liquid or bleach (or Cal-Hypo if the CH isn't too high, but note that for every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness by at least 7 ppm).

5) You can dilute the water to prevent the CYA from climbing as rapidly. This occurs naturally if you backwash your (sand) filter regularly or have summer or winter rains that overflow the pool. If water is cheap in your area (and not a precious resource) then intentional partial drain/refill can keep CYA in check.

6) You can use a saltwater chlorine generator (SWG or SWCG) to generate chlorine from salt in your pool.

Or you can use some combination of the above such as using Trichlor pucks/tabs, shocking weekly with chlorinating liquid, using PolyQuat 60 weekly or a phosphate remover, and doing some water dilution periodically. You can learn more about how to manage your pool by reading the Pool School.

[rmiller]

Hey Chem geek,

I'd like to say thanks for all the info I've used over the past 3 years. I installed a 18x36 IG pool 3 years ago and have enjoyed it pretty much ever since. I bought a pool kit and did everything (other than digging the hole and poolcrete).

This post caught my eye, since you state that a properly maintained FC/CYA level pool does not need to be shocked.

I installed my pool in July/Aug, closed in Oct., reopened in April and by June my CYA was around 100. I switched to liquid chlorine for 2 years and have only recently returned to using trichlor pucks as my CYA had gone down to about 40.

Just last night, in fact, I dumped in 2 gallons of 10% to do my bi weekly shock. Probably got the FC level to 12-14 since I started at 3 FC.

My pool water is well maintained, I test daily if I'm swimming, and this time of year I jump in almost every day. I have the k2006 equivalent kit, and because i test so much, I come to know how the pool reacts to certain things.

I had always assumed that the shock was to prevent the occurance of super bugs, and I always thought this seemed reasonable, not so much the super bugs part, but to give the water a re-set every couple weeks, seemed like a good idea.

Can you elaborate, or just confirm what your saying? I could easily quit the shock treatment, since I keep a pretty close eye on everything else.

I work with a lot of pool owners and have always thought the trichlor/dichlor addition of CYA was really overkill. Most people don't test for it, and you can really get it high when you don't have a lot of water turnover.

[chem geek]

I had always assumed that the shock was to prevent the occurance of super bugs, and I always thought this seemed reasonable, not so much the super bugs part, but to give the water a re-set every couple weeks, seemed like a good idea.

Can you elaborate, or just confirm what your saying? I could easily quit the shock treatment, since I keep a pretty close eye on everything else.

Chlorine is not like an antibiotic or a drug that kills pathogens through a very specific chemical pathway where mutations to work around that mechanism can lead to resistance. Chlorine is a broad-spectrum oxidizer where the chlorine attaches to a wide variety of nitrogenous compounds interfering with their function and oxidizes many of them breaking their bonds. It completely oxidizes ammonia, urea and other compounds to nitrogen gas, carbon dioxide and water. It reacts with many amino acids which are building blocks of proteins and reacts with many proteins for that reason as well as DNA (see this link for a more detailed discussion). Our skin, of course, is also exposed to this assault, but the surface is mostly dead skin cells that are being assaulted by other environmental factors as well and are sloughed off with new cells regenerated. Chlorine doesn't make it very far into our skin precisely because it reacts so readily with many nitrogenous compounds.

While it is true that using too low a level of chlorine will allow for heartier pathogens (species) to survive while their weaker brethren will be killed, it does not generally lead to mutations to create more resistant species. This is mostly because it takes an incredibly low level of chlorine to kill most bacteria faster than they can reproduce. The higher levels of chlorine used in pools are primarily to control algae growth since it takes more chlorine to kill algae which is a single celled plant and more complex with a thicker cell surface than bacteria. Most heterotrophic bacteria are killed at a 99% rate in one minute with 0.04 ppm FC where a 50% kill rate over the 15 minutes that is the fastest generation (doubling in population) rate is only 0.0004 ppm. The 50% kill rate for green algae over the 3 hours that is the fastest generation rate under ideal conditions (i.e. plenty of algae nutrients of phosphates and nitrates but with normal sunlight and temperature) is probably somewhere around 0.03 ppm. Pools with an FC that is 7.5% of the CYA level are equivalent in active chlorine (hypochlorous acid) concentration to having an FC of 0.06 ppm with no CYA.

Hypochlorous acid (shown here) looks a lot like water (shown here) so it becomes difficult to preferentially identify and "pump" this molecule out of cells. About the only significant single-point mechanism that can be used against hypochlorous acid is to have it react quickly with a reducing agent. The nitrite ion referred to in the paper I linked to above is an example of that, but the amount of reducing agent that must be produced by cells has to be enough to extinguish the hypochlorous acid at a rate faster than it enters into the cell and that can be very difficult.

If you maintain the appropriate FC level for your CYA level at all times, then there is no need for any regular shocking of the pool. You only shock when there is an unusual event such as vomit or fecal matter or a dead animal or significant measurable Combined Chlorine (CC) > 0.5 ppm or an overnight FC drop > 1 ppm or signs of algae, etc.

[birdslikemypool]

thanks everyone for your great help!

i did a few things and i'm already seeing the benefit.

first i took out about 1/3 to 1/2 of the pool water. then i added in some new water.

after that i got the ph down more. i think it was on the high end of the ideal range. i brought it down to the low end of the ph ideal range.

i've also been keeping both of the floating things full of chlorine tablets.

i haven't seen any algae yet.

[chem geek]

i've also been keeping both of the floating things full of chlorine tablets.

The chlorine tablets (assuming they are Trichlor pucks/tabs) will increase Cyanuric Acid (CYA aka stabilizer or conditioner) over time which will reduce chlorine's effectiveness allowing algae to grow. Read this post of mine above for options you have to handle this. For every 10 ppm Free Chlorine (FC) added by Trichlor, it will increase CYA by 6 ppm. So even with a low 1 ppm FC per day chlorine usage, 6 months of Trichlor will increase CYA by over 100 ppm if there is no water dilution. So you are going to have to do something additional to prevent algae growth -- keep the FC higher as the CYA rises or dilute the water to keep the CYA in check or use chlorinating liquid or bleach to prevent the CYA from rising or use a supplemental algaecide or phosphate remover or cross your fingers and hope for the best.