In it's natural state, chlorine is a gas. Many large commercial pools actually use gas injection systems to chlorinate their pools.
Now, to change chlorine into something we can use at home it needs to be chemically bound to something to turn it into a solid. The "somethings" that are commonly used are stabilizer (also known as CYA), calcium, lithium, or --- get this water.
All of these add a little salt to your water, but they add something else. Cal-Hypo add calcium, Tri-Chlor and Di-Chlor (tabs and most granules) add stabilizer, Lithium hypochlorite adds lithium and liquid chlorine adds - water.
All of these things can be bad for your pool (except the water) in large quantities. The stabilizer helps shield the chlorine from UV degradation, but at higher levels it also impairs the ability of chlorine to do it's work. The higher the stabilizer level you have the higher the amount of chlorine you need. Too much calcium and you start to get scaling on the walls and floors of your pool.
So, why do pool stores push these products? Several reasons.
- Money would be the first. Unless a pool store is in Florida, Arizona or other year round areas they must make their profit in a short swim season. So, they need to sell you as much as they can as quickly as they can. Additionally, chemical sales is their bread and butter. Profit on a bucket of tabs is much higher than on a gallon of liquid.
- Secondly, we are an immediate gratification society. We want a magic potion that will fix our problem right now. This is where the industry has tried to ad items like clarifiers, floculants and the like which in a perfect world help get the bad stuff out of the water quickly.
- Third in my book is training. Most pool store employees learn on the job or through seminars taught by chemical salesmen. So, bad information is handed down from employee to trainee and the chemical salesmen teach them to push high profit items. This is especially true in large chain stores where employees are paid commission and managers jobs are based on how much product flows out the door.