cloudy water and green algae

[crispy1985]

I can not get my pool clear. For a week my pool water has been cloudy with green algae on the bottom. I keep vacuuming but it is difficult to see the bottom due to the cloudiness. I have never had this problem before so I am not sure what to do or if what I have been doing has just added to the problem. I kept adding chlorine thinking that would kill the algae but all it has done is raise the chlorine level way too high! The test kit shows ph slightly low - I added ph, Still no change.
Do I need an algaecide or what should be my next step?

[xander]

we have the same problem in our pool right now; we keep adding chlorine and ph. chlorine level is quiet high the color is above 5.0 already and the ph is also very high 6.8 but still nothing hapend. the water still cloudy and green. though that is an algae but when we are doing the vacuum there is no green algae. we dont know what to do right now, we need someone can advice us, we been fighting this problem for one week already. may be there is problem about the minerals of the water? we dont know.

[chem geek]

Get a good test kit that can measure the Cyanuric Acid (CYA) level and read Defeating Algae. If your pool's CYA level is very high, then you'll have to do a partial drain/refill to lower it anyway and you'll need less chlorine to kill the algae after the CYA is lowered. Note that shocking a pool to kill algae is 1) not a one-time event -- you should MAINTAIN a high FC level and 2) the FC level needed is a function of the CYA level and is far higher than just 5 ppm or so.

[mdpool]

First off, I had a problem similar to that of the post that described having algae and adding chlorine to no avail. After I looked into the subject more i came to realize that my chloramines were to high. I use a LaMotte test kit with which I am able to test for chloramines. Anyhow, before doing anything make sure that chloramines arent the issue. If not and its just a normal case of algae what i do is add the appropriate amount of shock for the pools size to raise the ppm to that of 3ppm, then after letting that circulate throughout the system for approx. fifteen minutes I add a product called Yellow Out (2# treats 15,000 gallons) throughout the pool but especially where algae is the most apparent. Let circulate for 24-48 hours

[chem geek]

3 ppm FC is nowhere near enough to kill algae faster than it can grow, unless the pool doesn't have Cyanuric Acid (CYA) in the water. Since most outdoor pools have CYA, you need to shock the pool with an FC that is 40% of the CYA level if you want to kill the algae quickly and this high FC needs to be maintained -- not a one-time addition.

Yes, you could add a yellow treat product (or "No Mor Problems") which is sodium bromide that turns your pool temporarily into a bromine pool and since bromine doesn't bind to CYA it is able to kill algae at far lower levels (1-2 ppm). Yellow Out from Coral Seas is EDTA (normally used as a metal sequestrant). However, the fundamental reason that algae formed in the first place was that the FC/CYA ratio was too low, usually because the CYA was too high. In a manually dosed pool, the FC target minimum should be 7.5% of the CYA level. In an SWG pool, one can go as low as an FC that is 4.5% of the CYA level. The Pool School gives a lot more information including a chlorine/CYA chart for keeping a pool free of chlorine without the need for additional costly algicides, phosphate removers, or even regular shocking. My 16,000 gallon pool shown here is maintained with 12.5% chlorinating liquid alone plus a small amount of acid every month or two and it costs me $17 per month. The chlorine usage is low due to the opaque pool cover, but the pool is used 1-2 hours every day (more on weekends) and has a little less than 1 ppm FC per day chlorine demand. I use no algicides, no clarifiers, no phosphate removers, no weekly shocking, but I do maintain an FC appropriate for my pool's CYA level.

The chlorine/CYA relationship has been known since at least 1974 and was definitively determined in this paper. You can see how even a pool filled with lots of algae on spring opening can be cleared using bleach alone here. Yes, you can spend more on additional products, but chlorine alone can do the job.

[nocturnalsheep]

You can't assume that the algae growth was caused by the CYA/chlorine relationship...that's only one possibility. I have seen algae growth in pools with low CYA (30-50) and 9 ppm free chlorine. Algae growth can be spurred by many different factors.

I would recommend (if you have a sand filter) using a flocculant to sink the algae to the floor and then vacuuming it to waste (out the drain line). Physically removing algae is always the best choice because when you kill it, all the 'algae food' gets released back into the water to feed the next generation of algae.

Oh and Xander, 6.8 is a low pH, not a high pH

[chem geek]

There are over 20,000 pool owners at The PoolForum and over 10,000 at Trouble Free Pool plus hundreds of others on other forums including this one and we keep track of the water chemistry reported by all of them. Nearly every pool is able to control green algae by maintaining a minimum FC/CYA ratio (usually 7.5% in manually dosed pools; 4.5% in SWG pools). Yellow/mustard algae is harder to kill and needs a higher ratio to control its growth (usually 15% in manually dosed pools) though it's usually better to get rid of it completely.

The CYA test is done improperly by many pool stores and the test strips are often quite inaccurate compared to the turbidity test in the Taylor (and similar) kits. A 30-50 ppm range sounds like something from a test strip. What kind of test was used when you measured 9 ppm FC with 30-50 ppm CYA and what kind of algae was growing in how many pools and what was their primary source of chlorine (i.e. Trichlor tabs, chlorinating liquid, Cal-Hypo)? Higher levels are needed to shock a pool that already has algae (FC that is 40% of the CYA level for a reasonably quick clearing), but we're talking about maintenance levels in a pool with no visible algae and insignificant (< 1 ppm FC) overnight loss.

If the pool is a high bather load pool, such as a commercial/public pool, then there are some additional organic substances that can act like CYA and therefore reduce the active chlorine (hypochlorous acid) level that kills algae. At the other extreme, such pools can sometimes have combined chlorine, specifically monochloramine, in them that is able to kill algae in spite of CYA because monochloramine does not bind to CYA.

Algae is limited in its growth by nutrients, such as nitrates and phosphates, and by sunlight and temperature. A low FC/CYA ratio does not "cause" algae, but if the other conditions for algae growth are present, then too low an FC/CYA ratio allows for the algae to grow faster than the chlorine can kill it. However, even with all the nutrients the algae could want, its growth is still limited by sunlight and temperature and can only double in population roughly every 3 to 8 hours so if the FC/CYA ratio is high enough, it will kill much more than half the algae faster than it can double in population hence preventing it from growing. We have seen many pools with high nutrient levels still be able to control algae by chlorine alone. My own pool has 2000-3000 ppb phosphates and yes, it makes the pool quite "reactive" if the chlorine level gets too low, but maintaining the FC/CYA ratio works quite well. If one wants some additional algae prevention without side effects (such as staining from copper), then one can use 50 ppm Borates or use a weekly PolyQuat 60 algaecide though this is optional (and costs more) so is more like insurance.

Richard