The earlier you catch algae, the easier it is to get rid of. When algae is just starting, the water will have a dull appearance and the FC level will start falling more quickly than usual. If you catch algae at this point, it usually only takes a single application of chlorine to wipe it out.
Once you have a serious algae problem, there are two crucial things to understand about getting rid of it. First, algae is constantly growing. If you skip a day before you have gotten rid of it all, it starts growing right back. Skip a couple of days, and any time and chemicals you invested up to that point will be wasted.
When you first start fighting algae, it is a race: you need to kill the algae faster than it grows back. The earlier you catch an algae bloom, the easier it will be to kill. Or, if the algae is already well established, the harder you hit it at the beginning, the faster the entire process will go. Once you have algae on the ropes, things get a lot easier. At the end, you need to be thorough to make sure you got it all.
Second, the FC level you need to target depends on the cyanuric acid (CYA) level. The higher your CYA level the more FC it is going to take. If you don't have any idea of your CYA level, you risk either not using enough chlorine, and failing to get rid of the algae, or using too much chlorine, and causing metal corrosion and shortening the life of your vinyl liner (if you have one). If your CYA level is too low, most of your chlorine will be lost to sunlight, instead of fighting algae. If your CYA level is too high, you will need huge amounts of chlorine, which rapidly becomes impractical.
- Make sure you have enough chlorine on hand and enough time to get a good start on killing the algae. It is best to use either bleach or liquid chlorine.
- SLAM your pool; see SLAMing Your Pool for details. Try to do the first several cycles of testing FC and adding chlorine to shock level at hourly intervals, so you get a good start on killing the algae.
- While SLAMing, brush the entire pool every day.
- Monitor the filter and backwash/clean it as needed. DE filters are most likely to require frequent attention.
Once the algae is dead it will often turn gray or milky white. It can still take the filter a week, or more, to clear up the water from this point. You should see a visible improvement in the water each day. If you don't see a daily improvement, the algae might not all really be dead, or there might be a problem with your filter.
Algaecides rarely help and often cause problems. Copper based algaecides can cause unsightly copper stains that are difficult to remove, and can turn blond hair green. Linear quat based algaecides can cause foaming and bad smells. Polyquat based algaecides can be used as a preventive measure, but are ineffective at fighting algae once you have it. Polyquat is particularly useful when you are unable to maintain the FC level, such as over the winter or during an ascorbic acid treatment.
Reducing the phosphate level can help prevent algae. However, maintaining the proper FC level, which you really ought to be doing anyway, works just as well and costs far less. Don't be misled by aggressive sales tactics into buying an expensive phosphate treatment that you don't need.
For a more thorough write up on fighting algae, and directions on dealing with a pool that has been abandoned for some time, see Turning Your Green Swamp Back into a Sparking Oasis. To learn about adding borates to your water to help prevent algae see So you want to add borates to your pool--Why and How.
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