HERE IS A BASIC CHEMISTRY LESSON:
Sanitizer
After the alkalinity & pH are brought into range, the next adjustment is the sanitizer. Liquid Chlorine or Bleach is the most common and least expensive of the available sanitizers. The acceptable range is 1.5 - 3 ppm, though 4 - 5 ppm is absolutely fine. A commercial spa should be maintained at much higher levels - 5 - 6 ppm.
A residential pool should be maintained at 3.0 ppm. The higher levels ( 4 - 5 ppm) should be established, prior to a large party or heat spell. Try not to allow the levels to drop below 3 ppm, as this will prevent the need to shock or take other corrective action after the party.
ALKALINITY
There are a number of factors that contribute to the "balance of the water." The most important is the pH and the alkalinity. The alkalinity is the ability of the water to buffer the pH (or buffer chemicals that may affect the pH). The alkalinity is like an auto-pilot or cruise-control for the pH.
If the alkalinity is out of the acceptable range, then any small factor will cause a wild swing in the pH. This is called "pH bounce." Because of this, the alkalinity is the first thing that should be brought into line when balancing the water.
An ideal range for the alkalinity is 100-120 ppm (though 80 - 140 ppm is acceptable).
pH 7.4-7.6
The pH of the water determines if it is acidic or basic. Either extreme can have disastrous effects on the pool finish, equipment, or plumbing. When the pH is out of range, bathers may complain of eye discomfort or other ailments.
An acidic pH will cause the water to become aggressive. It will etch the plaster and cement finishes around the pool. Worse yet, it will dissolve the metallic components of the pool. The most expensive metallic component is the copper heat exchanger within the pool heater.
The ideal range for the pH is 7.4 - 7.6 (though 7.2 - 7.8 is acceptable).
HIGH OR LOW TOTAL ALKALINITY:
Total Alkalinity will determine the speed and ease of PH change.
If the starting PH is 7.6 in a 15,000-gallon pool and the Alkalinity is 120 ppm, if you add 1 pint of Muriatic Acid it will drop the PH to 7.0 and the Alkalinity to 105.
If the starting PH is 7.6 in a 15,000-gallon pool and the Alkalinity is 60 ppm, if you add 1 pint of Acid it will drop the PH to 6.7 and the Alkalinity to 26ppm.
So you see it is very important to keep the Alkalinity in the good range of 80-120 ppm. Alkalinity basically determines the waters ability to neutralize the acid.
Low alkalinity will cause:
Etched Plaster
Corroded Metals
Stained plaster
PH to creep, drift lower
Eye & skin irritation
High total Alkalinity will cause:
Scale formation
Cloudy water
Difficulty changing PH or the PH will drift up
Eye & skin irritation
So to adjust high alkalinity down you want to use Muriatic acid or Dry acid. When doing this it is important to test the acid demand before starting to lower the total Alkalinity. Never add more than ½ gallon of acid and don't add acid more than 3 times in one day.
If the total Alkalinity is high in a matter of hours the PH will rise. This happens because high total alkalinity will neutralize the acid. High alkalinity will create a constant high acid demand.
RAISING Total Alkalinity:
use SODIUM BICARB (Alkalinity Up {Baking Soda]) when you want to raise Alkalinity without raising the PH. Soda Ash will raise both at the same time. 20 oz of Alkalinity Up will raise your Alkalinity by 10 ppm (10,000 gallons).
One final note, the type of Sanitizer you use will determine your total Alkalinity level.
If you are using Trichlor 3" tablets, gas chlorine or bromine, the alkalinity should be kept in a higher range as each time these are added it will lower total alkalinity.
A neutral sanitizer like Dichlor (granular chlorine), will not raise or lower total alkalinity so you should try and keep it at 100 ppm.
High PH sanitizers like liquid Chlorine and Cal-hypo (shock) will raise your PH and total alkalinity so you should keep it in the low range 80-100 ppm.
Cyanuric Acid (Conditioner)
You add this to your pool water to protect the chlorine from being burned off rapidly from the Sun's UV rays. without Cyanuric acid (CYA) in your pool water, the chlorine will only last a matter of hours. With the proper level, the chlorine can last 5-10 times longer.
The ideal range is 30-50 ppm although it is okay if it is 80-100 ppm. Any higher and the chlorine starts to become less effective and you will need larger amounts of sanitizer in the water to maintain the chlorine level.
Note that Tri-chlor tablets and Dichlor will both add significant amounts of CYA to your pool water. Limit the use of these or your CYA can rise above 150 ppm or higher in one season.
Calcium Hardness
This is mainly determined by your water source. If your fill water is hard then the pools calcium hardness will be high. The Ideal range varies with surface type but 200-400 ppm is acceptable.
If your Hardness is low:
Water balancing will be difficult
Etching of plaster
Corrosion of pool equipment
Eye & Skin irritation
Foaming
To raise the calcium hardness add calcium chloride
High Hardness:
Scale formation on pool and equipment
Cloudy water
Eye & skin irritation
Water balancing will be difficult
To lower calcium hardness you will need to drain all or some of the water and refill the pool.