Hot Tub & Spa Water Chemistry 101 — A Complete Owner's Guide

---

 Four numbers govern almost everything about how your spa water behaves: pH, alkalinity, calcium hardness, and sanitizer level. Understand how these four interact and you understand spa water chemistry — the rest is mostly maintenance schedule.

pH measures how acidic or basic your water is, and it directly controls how well your sanitizer works. Too low, and chlorine burns off fast and irritates skin and eyes. Too high, and chlorine becomes largely ineffective even at a "correct" measured level — it's present in the water but not doing its job.

Alkalinity is the buffer that keeps pH from swinging. Low alkalinity means pH bounces around unpredictably with every top-off, bather load, or chemical addition — you'll find yourself constantly "chasing" pH without success. High alkalinity pushes pH upward and can cause cloudy water and scaling even when pH itself was recently adjusted.

The practical rule: always correct alkalinity before adjusting pH. If alkalinity is off, pH won't hold steady no matter how precisely you dose it.

This measures dissolved calcium in the water. Too low ("soft water") corrodes seals, gaskets, and metal components. Too high ("hard water") causes scaling on surfaces and inside plumbing, which restricts flow over time. Unlike pH and alkalinity, calcium hardness isn't something you adjust frequently — it's mainly a factor at fill time and worth testing after any major top-off.

This is what actually kills bacteria and breaks down contaminants. See our chlorine vs. bromine comparison for which fits your situation. Whichever you use, the sanitizer's effectiveness depends entirely on pH being in range — this is why "add more chlorine" often isn't the fix when water quality drops; correcting pH first often resolves it without adding anything.

A common scenario: sanitizer reads correctly, but water is cloudy and has a strong odor. The instinct is to add more sanitizer. But if pH is high, the sanitizer already in the water isn't working — adding more doesn't fix a pH problem. Checking and correcting alkalinity and pH first, then re-testing sanitizer, is the right order of operations almost every time.

Total Dissolved Solids isn't something you balance — it's a ceiling. It represents everything that's accumulated in the water over time (used chemicals, oils, cosmetic residue) and it only goes up until the water is drained. As TDS climbs, water becomes progressively harder to balance regardless of how correctly the other four numbers test. Once it nears 1500 ppm, no chemical adjustment fixes cloudiness — the water needs to be replaced.

1. Test alkalinity, pH, and sanitizer 2–3 times a week
2. Check calcium hardness after any significant top-off or refill
3. Track TDS periodically, especially with heavy use
4. Shock the water on a regular interval, not just when something looks off
5. Drain and refill (a Spa RepHresh) every 4–6 months, sooner if TDS is climbing fast

Water chemistry isn't complicated once you understand which number drives which problem — it's mostly about testing in the right order and not treating symptoms (cloudiness, odor) without checking the actual cause first. If you'd rather have someone track this for you, reach out and we can help get you dialed in!

About 95% of the spas we service in the Phoenix Valley run on chlorine. The rest use bromine. Neither is a "better" chemical across the board — they trade off differently, and which one fits your situation depends on a few specific factors.

Chlorine (granular, not tablet — tablets are formulated for pools and can over-dose a smaller spa volume) oxidizes contaminants like body oils, sweat, and lotion residue on contact, and kills bacteria in the process. It reacts fast, which is its main advantage.

Bromine kills bacteria the same way but reacts more slowly, and it remains active across a wider pH range than chlorine does — meaning it doesn't lose effectiveness as quickly when your water chemistry drifts slightly out of range.

• Speed. It breaks down contaminants faster, which matters if your spa sees heavy or frequent use (multiple bathers per day, Airbnb turnover, etc.) — you need a sanitizer that keeps up.
• Cost. Granular chlorine is generally less expensive per treatment than bromine.
• UV stability. Chlorine holds up under direct sunlight better than bromine does, which matters for spas that aren't always covered.

Its main downside is smell. What most people describe as "too much chlorine" is usually the opposite problem — chloramines, byproducts formed when chlorine binds to contaminants it hasn't fully broken down yet. The fix isn't less chlorine, it's more consistent shocking and better water balance, which clears the chloramines rather than masking them.

• Gentler on skin and eyes, particularly for users sensitive to chlorine.
• More forgiving of pH swings — it keeps sanitizing even when your water chemistry isn't perfectly dialed in.
• Milder odor overall, even when working correctly.

The tradeoffs: bromine breaks down contaminants more slowly, which can be a real issue in high-use spas where oils and lotions build up faster than a slower-acting sanitizer can keep pace. It's also incompatible with UV systems — bromine isn't stabilized against direct sunlight, so it degrades quickly in exposed water or with a UV-equipped spa, undercutting the point of having UV installed in the first place. And it's generally trickier to balance around oils and lotion residue, requiring more attentive dosing.

• Choose chlorine if: your spa sees frequent or heavy use, you have a UV or ozone system installed, or cost-per-treatment matters to you.
• Choose bromine if: you or household members have chlorine sensitivity, your spa sees lighter/occasional use, and you don't have a UV system that bromine would conflict with.

We use chlorine-based treatments as our standard approach because it holds up better across the range of usage patterns we see, and because properly balanced chlorine — not over-dosed, not under-shocked — doesn't produce the harsh smell or irritation people associate with it. If you're dealing with odor or irritation issues on your current sanitizer, the cause is more often imbalance than the chemical choice itself. Reach out and we can take a look at what's actually happening in your water.