Pool Chemicals: What You Actually Need

A pool store carries an intimidating wall of bottles. The water only cares about about six of them, and four of those exist to make the fifth one work.

The six that matter

Every one of those doses is written per 10,000 gallons. Run your pool through the pool volume calculator once, write the number on the inside of the shed door, and every label instruction from then on is a multiplication rather than a guess.

Why five of the six exist to serve the sixth

Chlorine is the only chemical in the list that sanitizes. The other five are there because chlorine’s effectiveness depends almost entirely on the water around it.

Free chlorine exists in two forms, and only one of them is the strong sanitizer. Which form dominates is set by pH: around pH 7.5 roughly half of your free chlorine is in the active form, and by pH 8.0 that has fallen to about a quarter. So a pool showing a perfectly respectable 3 ppm at pH 8.2 is doing considerably less work than the test strip suggests.

That is the whole argument for pH discipline. You are not balancing water for its own sake; you are deciding how much of the chlorine you paid for is allowed to function.

The order to add them

One at a time, with the pump running and time between each. The sequence exists because each step changes the ground under the next one:

1. Total alkalinity - it sets how stubbornly pH will resist you, so fix it first or you will fix pH twice.
2. pH - into the 7.4 to 7.6 range.
3. Calcium hardness - only if it is low, and it will not need touching often.
4. Stabilizer - only if it is low, and only outdoors.
5. Sanitizer - last, into water that is now able to make use of it.

⚠️ Never combine two pool products. Not in a bucket, not in a scoop, not in a puddle on the deck. Acid and chlorine together release chlorine gas. Use one bucket per product, rinse it, and leave time between doses.

Choosing a sanitizer

Tablets (trichlor) are the default for good reason: they sit in a floater or feeder and dissolve slowly. The catch is that they are stabilized, so every tablet adds cyanuric acid, and cyanuric acid never leaves except by draining water. A pool run on tablets for several seasons often ends up with stabilizer so high that the chlorine reading looks fine and the water still goes green.

Liquid chlorine (sodium hypochlorite) adds no stabilizer and no calcium, which makes it the clean choice when either is already high. It is the cheapest chlorine per unit of sanitizer and the most awkward to store - it loses strength on the shelf, faster in heat.

Cal-hypo granules add calcium instead of stabilizer, which is either useful or a problem depending on your hardness reading.

A salt chlorine generator is not an alternative to chlorine - it makes chlorine from salt, on site. It buys convenience, costs a cell every few years, and pushes pH up continuously as a by-product, so budget for acid rather than tablets. See how to lower pH in a pool for why a salt pool trends alkaline.

What you can walk past

• Clarifiers and flocculants - a cloudy pool is nearly always a chemistry or filtration problem. Fixing the cause beats settling the symptom, and flocculant in particular means vacuuming to waste afterwards.
• Weekly algaecide - algae grows where sanitizer fell short. Hold the chlorine and you rarely need it. Keep some for closing.
• “Pool perfect” all-in-one bottles - convenient, vague, and they make it impossible to know which reading you just moved.
• Enzyme treatments - real chemistry, narrow use case, and not a substitute for anything in the table above.
• pH stabilizer products - what stabilizes pH is total alkalinity in band. That is baking soda, and it is cheap.

Testing, which is the actual skill

Chemicals are the easy half. Knowing what to add requires reading the water, and the two habits that matter are:

• Test before you dose, not after you notice a problem. Twice a week in season is enough for most pools.
• Test the full set periodically, not just chlorine and pH. Alkalinity, calcium hardness and cyanuric acid move slowly, which is exactly why they get ignored until one of them is badly out of range. Once a month in season, and once at opening, covers it.

Strips are fine for the routine check. A drop kit is more accurate and worth having for the readings you are about to act on. Most pool stores will test a sample free, which is a genuinely good deal for the slow-moving readings - just be aware you are standing in a shop while they read it.

Worked example

An 18 by 36 ft pool averaging 5 ft deep:

volume = 18 × 36 × 5 × 7.48 ≈ 24,200 US gallons

Call it 24,000 gallons, or 2.4 label doses. That single number is what turns “add 1 lb per 10,000 gallons” into “add 2.4 lb”, and it is the difference between balanced water and a season of chasing readings.

Then the routine itself is short. Pool maintenance sets out what to do weekly, monthly and at each end of the season, and what a service company charges to do it instead.