What Hot Tub Chemicals Do You Need?

You need five things. In the order you will use them: an alkalinity increaser, a pH adjuster, a sanitizer, a shock, and a way to test. That is the entire list for a first fill, and for most owners it stays the entire list for a long time afterwards.

The list, in order

1. Total alkalinity increaser (sodium bicarbonate). First, because alkalinity is the buffer that decides whether your pH correction holds. Target 80 to 120 ppm.
2. A pH adjuster — usually a decreaser (sodium bisulfate) to start with, because most fill water sits high. Target 7.2 to 7.8.^[source]
3. A sanitizer — bromine tablets in a floating dispenser, or dichlor granules. On bromine you also need a sodium bromide bank on a fresh fill.
4. A shock — non-chlorine (MPS) for weekly oxidizing, dichlor for a genuine contamination event.
5. Test strips matched to your chemistry. Without these the other four are guesswork.

A start-up kit covers most of this in one order, which is the one time a kit is genuinely the right purchase — see the starter kit comparison for what should be in the box.

Why alkalinity comes first, and not pH

This is the single most useful thing on this page. Total alkalinity is a buffer: it resists changes in pH. If alkalinity is low, pH moves easily, and every correction you make gets undone within days. People experience this as a tub that will not hold pH no matter how much acid they add, and they respond by adding more acid, which lowers alkalinity further, which makes pH move even more easily.

Fix the buffer first and the pH usually settles on its own. If you have adjusted pH three weekends in a row, stop adjusting pH and go read your alkalinity. The mechanics are on hot tub pH and total alkalinity.

What the shop will also try to sell you

Calcium hardness increaser. Real product, real problem, and completely unnecessary if your fill water already has calcium in it. Soft water genuinely does need it, because water with too little calcium pulls calcium out of grout, seals and heater components. Test before buying. There is no calcium reducer, so over-correcting means draining. Details on hot tub calcium hardness.

Stain and scale control (a sequestrant). Essential on well water or a supply carrying metals. Pointless on soft municipal water. A test strip settles it in fifteen seconds.

Clarifier. Useful as a finisher after the chemistry is right and the filter is clean. Useless before that, and it loads the cartridge for nothing. It should not be part of a first purchase.

Fragrance and aromatherapy. These add organic load to water whose main ongoing problem is organic load. If you want them, use them sparingly and expect to shock more often.

Algaecide. Algae in a hot tub means the sanitizer has failed. Fixing the sanitizer fixes the algae, and a separate product does not address the cause. We cover why on hot tub algae.

The order to add them in, on a fresh fill

1. Fill, and let the tub come up to temperature. Chemicals behave differently in cold water.
2. Test the raw fill water. This is the reading everything else is relative to.
3. Correct total alkalinity to 80-120 ppm. Wait a full circulation cycle.
4. Correct pH to 7.2-7.8.^[source] Wait a cycle, retest.
5. Correct calcium hardness only if the test says so.
6. Establish the sanitizer. On bromine, the bank first, then tablets.
7. Shock, then wait the time the label states before getting in.

Do not follow the order printed on the individual bottles. Several of them will tell you to adjust pH before alkalinity, which is backwards for the reason above. The full version with quantities is on first chemicals after a refill, and the target numbers are on the dosage chart.

One accessory that earns its place

A floating dispenser, if you are running tablets. Tablets dropped loose into a tub dissolve wherever they settle, usually against the acrylic, and that is how bleached patches happen. A floater keeps them moving and lets you meter the dissolve rate. Start with the vent nearly closed and open it gradually across a week while you watch the readings.

What all of this is for

Holding a sanitizer residual. That is the core of the CDC’s guidance on controlling Legionella in hot tubs,^[source] and it is the reason the five-item list is five items rather than one. The balancers exist so the sanitizer works; the shock exists so the sanitizer is not consumed by things it should not be dealing with; the strips exist so you know whether any of it is happening.

Where you see an industry standard cited for spa water, check which vessel it covers. ANSI/APSP/ICC-5 covers residential inground swimming pools,^[source] not spas, and the right consensus reference for hot tubs is the water-quality standard for public pools and hot tubs.^[source]