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Swimming Pool Water Chemistry Guide

One thing that often mystifies first time pool buyers is the ins and outs of pool chemistry. But to keep your pool safe and clean, a solid understanding of pool water chemistry can make your life a lot easier. This guide will cover things like disinfectants, alkalinity (pH), stabilizers, algae prevention and filter aids. Read on to learn how to keep your pool water properly balanced.


Understanding pH and how it works is probably the most important thing to know when it comes to pool chemistry. In many ways, it's the linchpin of your pool's chemistry, as an incorrect pH can have a domino effect on every other chemical in your pool.

pH is a measure of hydrogen ion (H+) concentration in water.  If you recall from your days in grade school, this tells you whether a substance is an acid or a base, and that's the same for pool water. The pH scale goes from  0 (strong acid) to 14 (strong base) with 7 being neutral.

Staying slightly alkaline is typically the preferred level, staying somewhere in the pH range of 7.4 to 7.6. This range tends to be the most comfortable to the human eye and also is the best range for avoiding corrosion of the pool itself and maximizing the use of free chlorine

If pH is too low (below 7)

  • Water becomes acidic
  • Chlorine residuals dissipate quickly
  • Irritation of human membranes (particularly the eyes) can be moderate to severe
  • Plaster walls become etched
  • Corrosion of mechanical and metal parts including the impeller, heater and fittings
  • Staining 
  • Rapidly lowering of alkalinity

If pH rises above an 8

  • Chlorine becomes less effective
  • Pool walls collect scales and become discolored 
  • Pool water becomes cloudy (see our cloudy pool water guide for more info)
  • Filter is overworked
  • Eye irritation may occur

pH Adjustment
To avoid the problems listed above, pH must be maintained between 7.2 and 7.8. To raise the pH you can use a substance known as soda ash (sodium carbonate). It's important to realize that you can use too much soda ash. It's recommended that no more than two pounds be used for every 10,000 gallons during a single treatment. Another necessity when increasing pH this way is to keep the pump running to make sure that the water is circulating so your next measurement is accurate before you make any more adjustments. For pools with chemical feed pumps, some people use caustic soda (sodium hydroxide) for raising pH. If you're not able to stabilize pH this way, you may have to raise the total alkalinity of the pool water. 

Raising pH with Soda Ash
(Follow the chart  below if you have low pH, circulate the water and run another test)

7.2-7.42/3 oz.3 oz.6 oz.9 oz.12 oz.1 lb.2 lbs.
7.0-7.23/4 oz.4 oz.8 oz.12 oz.1 lb.1 1/4 lbs.2 1/2 lbs.
6.6-7.01 1/4 oz.6 oz.12 oz.1 lb.1 1/2 lbs.2 lbs.4 lbs.
Under 6.71 1/2 oz.8 oz.1 lb.1 1/2 lbs.2 lbs.2 1/2 lbs5 lbs

If your pH is too high, you should obtain an acid demand test kit. To lower pH, you'll need to ad muriatic acid or sodium bisulfate. If your pool has a deep end, that's where you should add the acid (be careful, this is a volatile substance). Keep your distance from the pool walls or any fittings to avoid damaging them. Carefully add acid at the deep end of the pool. Try not to pour acid near pool walls or fittings. NEVER ADD WATER TO ACID, add the acid to the water. That rule is for your own safety, always follow it.
Conversion: If you're moving away from muriatic acid, the comparable amount of sodium bisulfate is 10 pounds is roughly one gallon of muriatic acid. 

Lowering pH with Muriatic Acid
(Follow the chart  below if you have high pH, circulate the water and run another test)

7.6-7.81 1/4 oz.6 oz.12 oz.18 oz.24 oz.1 qt.2 qts.
7.8-8.01 1/2 oz.8 oz.16 oz.24 oz.1 qt.1 1/4 qts.2 1/2 qts.
8.0-8.42 1/2 oz.12 oz.24 oz.1 1/4 qts.1 1/2 qts.2 qts.1 gal.
Over 8.43 oz.16 oz.1 qt.1 1/4 qts.2 qts.2 1/2 qts.1 1/4 gal.

Factors which affect pH:

Lowers pH Raises pH
Acid Soda Ash
Gas Chlorine Sodium Hypochlorite
Trichlor Chlorine Calcium Hypochlorite
Dichlor Chorine Caustic Soda
Rain WaterBicarbonate of Soda
AlumSwimmer Wastes
Organic LitterAlgae Growth
Make up WaterMake up Water

Total Alkalinity (TA)

In layman's terms, TA is is your pool water's ability to resist change in pH. Though similar to pH, TA isn't a measure of concentration of the hydrogen ion but rather the ability to neutralize them. TA is 

the result of alkaline materials including carbonates, bicarbonates and hydroxides - mostly bicarbonates. This acid buffering capacity helps prevent wild swings in pH when acid or alkali end up in your pool water. 

Proper TA range - 80 to 150 parts per million (ppm)

If total alkalinity is too low:
pH changes rapidly when chemicals or impurities enter the water (rain water, etc.) causing pH to plummet and causing all of the aforementioned damage associated with low pH. 

If total alkalinity is too high:
changing your pH becomes like pushing a boulder uphill. If your pH is too high and stays too high you'll run into issues like cloudy water, troubles disinfecting, scale formation and filter problems.

Raising TA - Total alkalinity can also be raised by the addition of sodium bicarbonate or baking soda. The rule of thumb here is that 1.4 lbs. per 10,000 gallons will raise total alkalinity 10 ppm.

Raising Alkalinity Using Sodium Bicarbonate

100.14 lbs0.7 lbs.1.4 lbs.2.1 lbs.2.8 lbs.3.5 lbs.7 lbs.
200.28 lbs.1.4 lbs.2.8 lbs.4.2 lbs.5.6 lbs.7.0 lbs.14 lbs.
300.42 lbs.2.1 lbs.4.2 lbs.6.3 lbs.8.4 lbs.10.5 lbs.21 lbs.
400.56 lbs.2.8 lbs.5.6 lbs.8.4 lbs.11.2 lbs.14.0 lbs.28 lbs.
500.70 lbs.3.5 lbs.7 lbs.10.5 lbs14.0 lbs.17.5 lbs.35 lbs.

In some cases, soda ash can be used to raise total alkalinity. Pound for pound, soda ash raises alkalinity 60 percent more than sodium bicarbonate and is cheaper than sodium bicarbiante. The problem with using soda ash to increase alkalinity is it drastically increases pH. This can cause cloudy water and scale formation. Soda ash should only be used to increase total alkalinity if you also need to increase the pH or if only small increases in alkalinity are needed.

Chart No. 4 - Raising Alkalinity Using Soda Ash

100.09 lbs0.44 lbs.0.88 lbs.1.32 lbs.1.77 lbs.2.21 lbs.4.42 lbs.
200.18 lbs.0.88 lbs.1.77 lbs.2.65 lbs.3.53 lbs.4.42 lbs.8.83 lbs.
300.26 lbs.1.32 lbs.2.65 lbs.3.97 lbs.5.30 lbs.6.62 lbs.13.25 lbs.
400.35 lbs.1.77 lbs.3.53 lbs.5.30 lbs.7.07 lbs.8.83 lbs.17.66 lbs.
500.44 lbs.2.21 lbs.4.42 lbs.6.62 lbs.8.83 lbs.11.04 lbs.22.08 lbs.

Chemical manufactures are now marketing a total alkalinity increaser which combines the effects of sodium bicarbonate and soda ash. The product sodium sesquicarbonate or sodium hydrogen carbonate affects total alkalinity more than sodium bicarbonate, but does not cause quite as much increase in pH as soda ash does.

Chart No. 5 - Raising Alkalinity Using Sodium Sesquicarbonate

100.13 lbs.0.63 lbs.1.25 lbs.1.88 lbs.2.50 lbs.3.13 lbs.6.25 lbs.
200.25 lbs.1.25 lbs.2.50 lbs.3.75 lbs.5.00 lbs.6.25 lbs.12.50 lbs.
300.38 lbs.1.88 lbs.3.75 lbs.5.63 lbs.7.50 lbs.9.38 lbs.18.75 lbs.
400.50 lbs.2.50 lbs.5.00 lbs.7.50 lbs.10.00 lbs.12.50 lbs.25.00 lbs.
500.63 lbs.3.13 lbs.6.25 lbs.9.38 lbs.12.50 lbs.15.63 lbs.31.25 lbs.

Lowering total Alkalinity - Total alkalinity can be lowered by adding muriatic acid or sodium bisulfate. Acid may be added in doses of up to 1 quart per 10,000 gallons. Total alkalinity tests and further required additions of acid can be made every 2 hours.

Chart No. 6 - Lowering Alkalinity Using Muriatic Acid

102.56 oz.0.8 pts.0.8 qts.1.2 qts.1.6 qts.2.0 qts.1 gal.
205.12 oz.1.60 pts.1.6 qts.2.4 qts.3.2 qts.1.0 gal.2 gal.
307.68 oz.1.2 qts.2.4 qts.3.6 qts.1.2 gal.1.5 gal.3 gal.
4010.24 oz.1.6 qts.3.2 qts.1.2 gal.1.6 gal.2.0 gal.4 gal.
5012.80 oz.2.0 qts.1.0 gal.1.5 gal.2.0 gal.2.5 gal.5 gal.

Chart No. 7 - Lowering Alkalinity Using Sodium Bisulfate

100.21 lbs.1.06 lbs.2.13 lbs.3.19 lbs.4.25 lbs.5.31 lbs.10.63 lbs.
200.43 lbs.2.13 lbs.4.25 lbs.6.38 lbs.8.50 lbs.10.63 lbs.21.25 lbs.
300.64 lbs.3.19 lbs.6.38 lbs.9.56 lbs.12.75 lbs.15.94 lbs.31.88 lbs.
400.85 lbs.4.25 lbs.8.50 lbs.12.75 lbs.17.00 lbs.21.25 lbs.42.50 lbs.
501.06 lbs.5.31 lbs.10.63 lbs.15.94 lbs.21.25 lbs.26.56 lbs.53.13 lbs.

Calcium Hardness:

Calcium hardness is a measure of the dissolved calcium salts in water. Under normal conditions this should not be a problem in properly operated swimming pools. Estimates of the proper range of calcium hardness vary widely but the ideal level for plaster pool is generally considered to be about 250 ppm. If calcium hardness is very low then water may leach calcium from pool walls causing pitting of the plaster surface. Very high calcium hardness may contribute to scale formation and clouding of the water. To raise calcium hardness - add calcium chloride.

Chart No. 8 - Raising Hardness With Calcium Chloride

102 oz.10 oz.1 1/4 lbs.1 3/4 lbs.2 1/2 lbs.3 1/4 lbs.6 1/4 lbs.
204 oz.1 1/4 lbs.2 1/2 lbs.3 3/4 lbs.5 lbs.6 1/4 lbs.12 1/2 lbs.
306 oz.1 3/4 lbs.3 3/4 lbs.5 1/2 lbs.7 1/2 lbs.9 1/2 lbs.18 3/4 lbs.
408 oz.2 1/2 lbs.5 lbs.7 1/2 lbs.10 lbs.12 1/2 lbs.25 lbs.
5010 oz.3 lbs.6 1/4 lbs.9 1/2 lbs.12 1/2 lbs.15 3/4 lbs.31 1/4 lbs.

To lower calcium hardness anhydrous trisodium phosphate may be used. One pound trisodium phosphate per 10,000 gallons will lower calcium hardness 11 ppm. Use in small increments or clouding may occur. Another method of lowering calcium hardness is to simply drain off part of the pool water and dilute the remaining water with fresh make up water.

Total Dissolved Solids (TDS)

After a pool has been in use for a time, dissolved solids may begin to accumulate. These unfilterable solids include body wastes, suntan lotion, stabilizer, chlorines, algicide, dirt, pollen, etc. Normally this is less of a problem with outdoor pools because of rain water and no use during winter months. Indoor pools sometimes have a buildup of dissolved solids requiring draining the pool and refilling with fresh water. Most pools should be drained after 3 to 5 years. Ideally pool water contains under 450 ppm total dissolved solids.

A proper balance of the previously described water chemistry factors will provide water that will not damage pool components and is non-irritating to swimmers. It is then necessary to provide for disinfection of the water to prevent the spread of disease organisms from person to person and prevent unwanted growth of bacteria and algae in the pool.

The most commonly used disinfectant for swimming pools is chlorine. In its elemental form chlorine is a heavy greenish yellow gas which is so toxic that is has been used as a weapon in chemical warfare. Because of the extremely high potential for injury or death from improper use of chlorine gas, a number of chlorine compounds have been formulated to provide chlorine in forms that can be handled and used safely by swimming pool operators.

The following forms of chlorine are commonly used in swimming pools.

Gas Chlorine: 100% available chlorine
Advantages: Disadvantages:
Cheapest form of chlorine Extremely dangerous
No residue from carriers Special room needed for chlorine
Feed equipment is expensive
Special training and safety equipment needed
Lowers pH, must constantly add pH increaser

Because of the special hazards associated with the use of gaseous chlorine, its use has been prohibited at public swimming pools in several states.

Calcium Hypochlorite: granular or pelletized 65% available chlorine

Relatively cheapNot stabilized - may lose strength if not tightly covered.
Can be mixed into solution for feed pumpsDoes not dissolve completely - leaves residue
Can be used in some specially designed erosion feedersDoes not dissolve completely - leaves residue
High pH (11.7) raises pH of pool
Highly reactive - may cause fires

WARNING - do not use in closed tablet feeders designed to use other forms of chlorine

Sodium Hypochlorite: Liquid bleach 12.5% available chlorine

Next to gas is the cheapest chlorine availableBulky and heavy
No dissolving required - no residueNot stabilized-loses strength rapidly
Can be used with chemical feed pumpsHigh pH (10-13) raises pH of pool

Trichloroisocyanuric Acid: sticks or tablets 90% available chlorine

Stabilized - chlorine doesn't dissipateCost slightly higher
Easy to handleLowers pH - pH 2.8
Low cost, low maintenance erosion feedersLowers total alkalinity
Highly concentrated - 90% available chlorineMay elevate cyanuric acid levels
Dissolves completely - very little residueNot suitable for supeprchlorination

Other forms of chlorine less commonly used are lithium hypochlorite, potassium dichloroisocyanuric acid, and sodium dichloroisocyanuric acid.

Free Chlorine Residual is the amount of chlorine in the pool which has not reacted with substances other than water. It is the chlorine which is available to disinfect pool water and oxidize organic substances. Free chlorine residual should be maintained between 1 and 3 ppm.

Combined Chlorine is chlorine in the pool which has reacted with substance other than water and is no longer available in its free state. Some combined chlorines are bactericides but they contribute little to the disinfection process. Chlorine combined with ammonia produces chloramines which cause eye irritation and an objectionable chlorine odor. For this reason combined chlorine residual should be kept to a minimum preferably below 0.2 ppm.

Total Chlorine residual is the concentration of free chlorine plus combined chlorine. To determine combined chlorine residual test for free chlorine and total chlorine.

Total chlorine - free chlorine = combined chlorine

Breakpoint Chlorination is the process by which combined chlorine and some organics are "burned out" of the pool by addition of large amounts of chlorine. The reaction of chlorine with ammonia to form chloramines occurs in several stages with free chlorine consumed at each stage. If enough chlorine is added to the water the total chlorine residual will rise to a point that forces the reaction of chlorine with ammonia to go rapidly to completion. Compounds of nitrogen and chlorine are released from the water and the apparent residual chlorine decreases. The point at which the chlorine residual suddenly drops is called the breakpoint. When enough chlorine is added to pass the breakpoint, combined chlorine compounds disappear, eye irritation potential and chlorine odors disappear, and the chlorine remaining in the water is all in the free state.

Superchlorination: In order to prevent buildup of chloramines in the pool it is necessary to periodically add large amounts of new chlorine in an effort to pass the breakpoint. Public swimming pools should be supechlorinated about once a week. The amount of chlorine needed to reach the breakpoint will vary depending on the amount of organic material introduced by bathers and on the level of free chlorine maintained in the pool. If the amount of combined chlorine is known then the amount of new chlorine needed is ten times the amount of combined chlorine. When combined chlorine residual is not known, superchlorination is accomplished by adding 10 ppm of new chlorine to the pool. Ordinarily calcium hypochlorite at a dose of at least 1 lb. per 10,000 gallons is used for superchlorination. The chart below shows the amounts of various chlorine compounds which can be used to introduce 10 ppm of chlorine to the pool.

Chart No. 9 - Superchlorination
(Amount Needed to Introduce 10 ppm)
Type of chlorineGALLONS IN POOL
Sodium Hypo10 oz.1 3/4 qts.3 1/4 qts.1 1/4 gal.1 2/3 gal.2 gal.4 gal.
Lithium Hypo4 oz.1 1/4 lbs.2 1/3 lbs.3 1/2 lbs.4 3/4 lbs.6 lbs.12 lbs
Dichlor2 1/4 oz.11 oz.1 1/3 lbs.2 lbs.2 2/3 lbs.3 1/3 lbs.6 3/4 lbs.
Calcium Hypo2 oz.10 oz.1 1/4 lbs.2 lbs.2 1/2 lbs.3 1/4 lbs.6 1/2 lbs.

Non-chlorine Shock Treatments
Several products have been developed which oxidize organics without the use of chlorine. Pools which use those products can accomplish the reduction of organics without closing the pool for any longer than it takes to dissolve and distribute the chemicals. Those products are more expensive than chlorine but may be preferred where it is necessary to keep a pool open.

How pH affects free chlorine residual Chlorine reacts with water to form Hypochlorous acid (HOC1). The reaction is different for each form of chlorine but hypochlorous acid is produced by each of those reactions and is the form in which chlorine serves best as a disinfectant. Hypochlorous acid is a weak acid and easily dissociates to an ionized hypochlorite state as shown below.

HOC1increasing pH->H+ OC1-
Hypochlorous<-decreasing pHHydrogen + Hypochlorite

This is important because both hypochlorous acid and the hypochlorite ion are counted as free chlorine residual on your test kit but only the hypochlorous acid portion is an effective disinfectant. The balance between hypochlorous acid and the hypochlorite ion is affected by pH. The higher the pH, the less hypochlorous acid present and the less effective free chlorine becomes. At a pH of 7.2 about 66% of free chlorine is hypochlorous acid. At a pH of 7.8 only about 33% of free chlorine is hypochlorous acid. Thus pH control is essential for maintaining the effectiveness of chlorine as a disinfectant.

Stabilizer - Cyanuric Acid
Hypochlorous acid is a highly unstable molecule which dissipates rapidly in the presence of sunlight. This results in considerable loss of free chlorine form pools exposed to sunlight. Proper stabilization of chlorine with cyanuric acid slows the rate of chlorine dissipation without appreciably sacrificing oxidation and disinfection activity. Reaction of free chlorine with cyanuric acid produces a form of combined chlorine (chlorimide) which is active enough to aid disinfection and show up as free chlorine residual on your test kit. Proper stabilization requires 30 to 50 ppm cyanuric acid. Outdoor pools should be initially treated with 40 ppm cyanuric acid. The chart below can be used to determine the amount of cyanuric acid needed.

Chart No. 10 - Establishing or Increasing Cyanuric Acid Level

101 1/4 oz.6 1/2 oz.12 3/4 oz.1 1/4 lbs.1 2/3 lbs.2 lbs.4 lbs.
202 1/2 oz.12 3/4 oz.1 3/4 lbs.2 1/2 lbs.3 1/3 lbs.4 lbs.8 1/3 lbs.
302 1/2 oz.11 oz.1 1/3 lbs.2 lbs.2 2/3 lbs.3 1/3 lbs.12 1/2 lbs.
403 oz.10 oz.1 1/4 lbs.2 lbs.2 1/2 lbs.3 1/4 lbs.16 2/3 lbs.
506 1/2 oz.2 lbs.5 1/4 lbs.6 1/4 lbs.8 1/3 lbs.10 1/2 lbs.21 lbs.

Cyanuric acid dissolves very slowly and is best predissolved in warm water before introducing into the pool. If it is not predissolved it may take several days to dissolve completely. Once added to the pool, cyanuric acid does not dissipate. It is removed from the pool only by splash out and backwash waste. Stabilized chlorine products such as trichloroisocyanuric acid will add stabilizer to the pool and may cause a gradual rise in cyanuric acid concentration. Excessive amounts of cyanuric acid can interfere with the disinfection process and at concentrations above 100 ppm may cause "chlorine lock" and clouding of the pool. Cyanuric acid level is lowered by draining part of the water out of the pool and diluting the remaining water with fresh water. Generally cyanuric acid level should be kept below 60 ppm. Stabilized forms of chlorine should not be used for superchlorination because cyanuric acid level may be increased.

Bromine is chemically very similar to chlorine. Bromine compounds tend to react more slowly than chlorine compounds so bromine is generally more stable and less subject dissipation in sunlight. The dissociation of hypobromous acid into the bromine ion is less affected by pH than the corresponding reaction of chlorine. This makes bromine active over a larger range of pH than chlorine. Bromine will combine with ammonia to form bromamines similar to chlorine but unlike chloramines, bromamines are effective bactericides and do not produce the degree of odor and eye irritation associated with chloramines. Bromine is less affected by high temperature and nitrogen wastes than chlorine so it is particularly attractive for use in hot water spas. Bromine is more expensive than chlorine and has not yet received widespread acceptance by swimming pool operators.

The form of bromine most commonly used in pools and spas is the organic chemical bromo-chloro-dimethylhydantoin which contains both bromine and chlorine. It is marketed under various trade names and is generally in tablet form for use in erosion feeders..

Bromine residual should be maintained between 2 and 4 ppm. Bromine residual is measured using the DPD #1 test used to measure free chlorine. If your test kit does not include a bromine scale then bromine residual is approximately 2.25 times the reading on the chlorine scale.

Biganide disinfectants
The only disinfectant other than chlorine and bromine which has been accepted as a primary disinfectant in public swimming pools is polyhexamethylene biguanide. Biguanide is used at a concentration of 30 to 50 parts per million and a pH of 7.2 to 7.8 to kill germs and control algae growth. A special test kit is needed to test the biguanide residual. The main advantage of biguanide is the disinfectant concentration remains fairly stable so it requires less frequent adjustment than chlorine. No automatic chemical feeder is needed.

Biguanide is not an oxidizer and will not destroy organic wastes the way chlorine and bromine do. It must be used in conjunction with a peroxide pool shock treatment to prevent organic wastes from accumulating in pool water. Biguanide is incompatible with chlorine and most algicides. Chlorine in make-up water can cause clouding of biguanide pools. Biguanide increases the staining potential of dissolved metals in a pool so copper based algicides, copper ion generators, and pool heaters should not be used. Only chemicals recommended by the disinfectant manufacturers should be used.

Supplemental disinfection equipment
A variety of supplemental disinfection process equipment is being marketed for use on swimming pools. The most common are copper/silver ion generators, ozone generators, and ultraviolet light generators. While each process provides some disinfection activity, they are not accepted as primary disinfectants in public swimming pools because they are either too slow or do not provide a disinfectant residual. Supplemental disinfection equipment, if used, must be used in conjunction with a free chlorine or bromine residual.

Free Chlorine, ppm1.01.0-3.03.0
Combined chlorine, ppmNoneNone0.2
Bromine, ppm2.02.0-4.04.0
Total Alkalinity, ppm6080-100180
(for Liquid Chlorine, Cal-Hypo and Lithium Hypo)
(for gas chlorine, dichlor, trichlor and bromine compounds)
Calcium Hardness, ppm150200-400500-1000+
Cyanuric Acid, ppm1030-50150
(except where limited by Health Dept. requirements, often to 100 ppm)

Suggested NSPI Standards - Spas

Free Chlorine, ppm1.01.0-3.010.0
Combined chlorine, ppmNoneNone0.2
Bromine, ppm2.02.0-4.010.0
Total Alkalinity, ppm6080-100180
(for Liquid Chlorine, Cal-Hypo and Lithium Hypo)
(for Gas Chlorine, Dichlor,Trichlor and Bromine Compounds)
Calcium Hardness, ppm150200-400500-1000+
Cyanuric Acid, ppm1030-50150
(except where limited by Health Dept. requirements, often to 100 ppm)


Algae are tiny plants that bloom and grow in swimming pools if nutrients are present and a sufficient level of free chlorine is not maintained. Below are descriptions of the three most common algae problems in swimming pools.

Green Algae The most common algae in swimming pool floats in water and coats pool surfaces. Left unchecked green algae will very quickly turn the pool water pea green.

Mustard Algae settles on pool walls and causes a slimy yellow film.

Black Algae appears in "buds" or clumps attached to tile grout, corners, steps and pool surfaces.

Green Algae - is very susceptible to chemical treatment (see algaecide). Superchlorinate with 10 to 20 ppm chlorine in the evening. Keep the filter running and brush the pool walls and bottom. Periodically check chlorine and maintain above 3 ppm until water clears. Using an algicide containing quaternary ammonia the next morning will help prevent the return of green algae.

Mustard Algae - is much more resistant to chemical treatment and clings more tightly to pool walls than green algae. Adjust pH and superchlorinate as for green algae then brush diligently. Later vacuum the pool, check chlorine and superchorinate again if necessary. Mustard algae will generally return unless treated with a special mustard algicide or a copper based algicide. Algicide should be added in the morning to treat algae in daylight - its most active period.

Black Algae - is very difficult to get rid of. It can be controlled to some extent by frequent superchlorination and diligent brushing with a stiff brush. Spot treatments can be made by turning off the recirculation pumps and pouring granular chlorine directly on recently brushed spots. Trichlor tablets can also be rubbed on recently brushed areas to spot treat. Black algae can usually be controlled with the use of strong alicides and maintenance of relatively high free chlorine residual, but complete removal of black algae may require draining and cleaning the pool.

Note: Algae blooms are a problem best avoided. Maintaining proper water quality and frequent brushing of pool walls will deprive algae of the opportunity to get started.