Enter whatever your test strip shows — leave the rest blank — and get every spa reading graded at once, with doses sized in teaspoons and your next drain date.
Doses are spa-sized estimates — your product's label always wins, and granule strengths vary by brand. Add one measure at a time with the jets running, then retest after 20 minutes. Small water punishes guessing.
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Why spa targets aren't pool targets
Copying pool numbers into a hot tub is the most common spa mistake, and three of them are genuinely different. Sanitizer runs higher: 3–5 ppm chlorine or 4–6 ppm bromine, versus a pool's 1–3, because 102°F water destroys sanitizer quickly and is exactly the environmentPseudomonas aeruginosa — the bug behind hot tub rash — likes best. Calcium runs lower: 150–250 ppm rather than 250–450, because hot water holds less calcium in solution and your heater element is the first thing scale finds. And stabilizer runs much lower: over 50 ppm in a spa means drain, where a pool would shrug it off.
Alkalinity before pH in a spa
This card deliberately puts alkalinity above pH in your fix order, which is the opposite of how most pool advice reads. The reason is your sanitizer: both dichlor and bromine are acidic, so they push pH down a little every single dose. With total alkalinity under 80 there's nothing buffering that, and you'll be chasing pH twice a week forever. Set the buffer near 100 first and pH mostly stops moving on its own.
The drain date is a chemical, too
Spa water has a shelf life that no chemical fixes, because dissolved solids, body oils, and stabilizer only ever accumulate. The industry shorthand is the water replacement interval: gallons ÷ 3 ÷ average bathers per day = days of usable water. A 350-gallon tub with two daily soakers works out to 350 ÷ 3 ÷ 2 ≈ 58 days. Heavy use shortens it fast — six people a day in the same tub drops it to about 19. Drain sooner if CYA has passed 50, and drain at least quarterly no matter how lightly it's used. On a spa this is a cheap fix: a few dollars of water and about forty minutes.
What the abbreviations mean
FC is free chlorine, the portion still available to sanitize. TA is total alkalinity, the buffer that keeps pH from swinging. CH is calcium hardness, how much dissolved calcium the water carries — too little foams, too much scales.CYA is cyanuric acid, the stabilizer that comes bundled into dichlor granules; useful in sunlight, useless under a spa cover, and a problem once it builds up. ppm is parts per million, which for water is the same as milligrams per liter.
Frequently asked questions
What chemicals does a hot tub actually need?
A sanitizer (either chlorine as dichlor granules, or bromine), something to move pH up and down, an alkalinity increaser (plain baking soda works), and — if your fill water is soft — calcium hardness increaser. That's the whole list. Clarifiers, enzymes, and weekly 'treatments' are optional extras that a properly sanitized spa doesn't need.
What should hot tub chemical levels be?
Free chlorine 3–5 ppm, or bromine 4–6 ppm — both higher than a pool, because hot water burns sanitizer off fast and the bugs that cause hot tub rash thrive in it. pH 7.2–7.8, total alkalinity 80–120 ppm (aim near 100 if you use dichlor or bromine, since both are acidic), and calcium hardness 150–250 ppm, which is deliberately lower than a pool's target.
How often should I drain my hot tub?
Use the water replacement interval: gallons divided by three, divided by the average number of people soaking per day — that's how many DAYS the water lasts. A 350-gallon spa used by two people daily comes out around 58 days. Regardless of the math, drain at least every three to four months, and drain immediately if your stabilizer (CYA) has climbed past 50 ppm.
Why is my hot tub foaming?
The usual culprit is calcium hardness that's too low — soft water plus jet agitation whips up foam the way soap does. Body oils, lotions, and detergent residue from swimsuits make it worse. Check your calcium first: below about 150 ppm in a spa, foaming is expected rather than mysterious.