CHLORINE TABS, POWDER AND LIQUID

By far the most popular way of getting chlorine in the pool.
You buy chlorine and use it in one of these three ways.

LIQUID CHLORINE

Liquid chlorine is "Sodium Hypochlorite".

Mostly used as a shock, liquid chlorine can be used as the primary sanitizer but getting it into the pool consistently to keep up with loss and demand makes it difficult for residential use.

You need a liquid pump system for it to be effective. Other than for shock then, liquid chlorine is primarily used only for commercial applications where they have big tanks to hold and pump it into the pool.

POWDERED CHLORINE

The powdered version of chlorine comes in various forms.

Chlorine can be manufactured from sodium or calcium and powdered versions are typically sodium di-chlor or cal-hypo (calcium hypochlorite).

Powdered chlorine is generally used for shocking the pool only due to the same issues as liquid. There is no daily delivery method besides you going out and dumping it in.

RIDING THE ROLLER COASTER

Because there is no good delivery system for liquid or powdered chlorine, most people using them as a sanitizer "ride the roller coaster".
Sanitizers are what you test for on the test kit you have.
There is a range you need to keep the sanitizer in for it to be effective.
Too low and you get stuff growing in the pool.
Too high and you irritate people, bleach out liners or prevent the pool from being used at all.
When you add just the right amount of chlorine, either liquid or powder, your test kit says good job.
Chlorine starts going away almost immediately and at some point your test kit says add more, probably the next day.
So to keep the chlorine in the good range you have to add it daily or twice daily or three times a day depending on use, rain etc.
No one does that. You dump a enough chlorine in to last three days or more and repeat. It's really high on day one and all but gone by day three.
This is the Roller Coaster.
High chlorine, low chlorine, up and down on and on.
Somewhere in the middle your test strip reads "ok" but before long, not ok.
This method guarantees one of two things;
You will spend more on chlorine because you add so much the test is always high or you will have to spend more to fix a problem because the chlorine got too low.

ENTER THE TABLET

If you want a reliable, steady delivery method without spending big dollars on a liquid chlorine tank and pump system, a tablet feeder gets the job done quite well for about a hundred bucks.
Yes you can throw a tablet in the skimmer or put it in a floater but those methods have no control over the amount of erosion that takes place.
No one tests the water and if the chlorine is too high, takes a half melted tablet out of the skimmer. What do you do with the thing after removal?
Much like the roller coaster, no reliable, adjustable delivery method means you use more product.
A tablet feeder allows control over the chlorine by turning a dial and increasing the amount of tablet erosion daily.
If the test says too high or low you turn the knob up or down to adjust the flow of water through the feeder.
This simple, inexpensive method gets the job done for just about any pool.

ENTER THE TABLET, DOWNSIDE

The biggest downside to the chlorine tablet is "over stabilization".

The vast majority of chlorine tablets contain "cyanuric acid" (CYA) aka stabilizer aka conditioner. These tablets known as "tri-chlor" utilize cyanuric acid to help the chlorine stabilize and stay in the water longer. And that's great, to a point.

As the CYA builds up in the water, beyond a certain point chlorine becomes less effective.

So you end up needing more chlorine and turning up the dial on the feeder also feeds more CYA and so you need more chlorine and so on.

Almost every pool that uses tablets becomes over stabilized requiring water to be removed, fresh water put in thereby diluting the CYA to a lower level.