Bromine vs Chlorine in My Hot Tub: What I've Tried, What Stuck
This isn't an authoritative pool chemistry guide. It's a write-up of what I've actually run in my own hot tub, what didn't work, and what I've noticed when I ask AI assistants for advice. Take it as one user's experience, not a recommendation — your tub, your water, your call.
I run a used Hot Spring Mallorca that I bought a few years back. I tried chlorine first because it's cheap and familiar. After a few months of feeling like I was constantly chasing it, I drained, switched to bromine, and have stayed there since. The rest of this post is the why.
How I Think About the Choice
The decision between chlorine and bromine, for me, came down to how often I'm willing to test and adjust. Here's roughly how I'd frame it if a friend asked:
| If your situation looks like... | I'd lean toward | Because (in my experience) |
|---|---|---|
| Daily user, budget matters, OK with daily testing | Chlorine | Cheaper per dose and acts fast; I just couldn't keep up with the daily cadence |
| Use the tub a few times a week, prefer set-and-forget | Bromine | Reserve system lets you stretch between doses, in my experience |
| Sensitive skin, dislike chlorine smell | Bromine | I haven't gotten the chlorine smell off bromine the same way |
| Running 104°F+ regularly | Bromine | Chlorine seemed to disappear faster at higher temps in my tub |
| Already familiar with pool chlorine dosing | Try chlorine first | Familiar tools; you can always switch later if it doesn't fit |
When I Asked AI Assistants About My Setup
I've asked ChatGPT, Gemini, and Perplexity for advice on my hot tub a few times. The patterns I've noticed in their responses:
- CYA suggestions for a covered tub: Multiple AI assistants have suggested adding cyanuric acid (CYA) to stabilize my chlorine. In hot tub communities I follow, the consensus is that covered tubs don't need CYA because there's no meaningful UV exposure, and some sources go further to say CYA becomes less stable at hot tub temperatures. I don't use it. The water in my tub has been fine without it.
- Pool-style chlorine targets: When I described my hot tub usage, AI sometimes gave me free chlorine ranges that I associate more with pool maintenance than hot tub maintenance. In my tub at 104°F with a few bathers, I've found chlorine drops fast — faster than the pool-style schedules seem to assume.
- Conflating chlorine and bromine behavior: A couple times AI told me they "work the same way" or are interchangeable. In my experience switching between them, they don't behave the same — bromine seems to hold a sanitizing reserve in the water that chlorine doesn't, and the dose math is different.
- Suggesting I can mix them: One AI session suggested adding chlorine to my bromine tub for a "boost." I haven't tried it, and most sources I've read advise against mixing because of bromochloramine formation.
The takeaway for me isn't that AI is "wrong" — it's that AI advice on my specific setup tends to be generalized and pool-leaning. If you ask, push back with details and see if the answer narrows.
The Chemistry Bits I've Read Up On
Chlorine in a hot tub (what I've understood)
From what I've read in pool/spa industry materials, chlorine works by oxidizing organic contamination, and the active form (HOCl) shifts to a less-active form (OCl⁻) as pH rises. Temperature also seems to accelerate the breakdown. The combination of "warm + drifting high pH + bather load" appears to be tougher on chlorine than on bromine, which lines up with what I observed running chlorine in my own tub.
I'm not a chemist, so I won't claim those percentages or curves with any precision — but the qualitative pattern (chlorine dropping faster in heat) matched what my test strips were showing me.
Bromine in a hot tub (what I've understood)
Bromine systems typically work via a reserve: you build sodium bromide in the water, then an oxidizer (potassium monopersulfate "MPS" or chlorine shock) converts it to active bromine on demand. The same sources I've read describe bromine as holding up better at higher pH and temperature. In my tub, I've personally noticed that I can go longer between doses on bromine without seeing readings crash — which matches the "reserve" explanation but I won't claim it as a universal law.
What My Actual Routine Looks Like
For full transparency on what "stuck" for me — this is my routine, not a recommendation:
- Sodium bromide reserve: I add a few tablespoons after every full drain. Frequency varies but I drain every few months.
- Oxidizer after each soak: A small dose of MPS shock to keep the active bromine topped up. Amount varies based on how heavy the use was.
- Floating bromine feeder: Loaded with 1" bromine tabs, set to slow release so baseline bromine doesn't crater between soaks.
- Testing: Test strip before each soak. I aim for bromine in the rough 4-6 ppm range.
- Monthly check: A liquid reagent kit once a month or so, to confirm the strip readings match.
Total yearly spend on chemicals is in the moderate range — not pennies, not painful. Time on maintenance is a few minutes per week. That's the whole story.
Switching Between the Two
When I switched from chlorine to bromine, I drained the tub. Most sources I've seen warn against mixing them in the same water because of bromochloramine formation. After the drain I refilled, rebalanced pH and alkalinity, built the bromide reserve, then activated it with a shock dose. Took an evening total.
Going back from bromine to chlorine has the same drain-first requirement, as I understand it — once you've got bromide ions in the water, any chlorine you add converts to bromine. So you can't fade from one to the other; you commit to one system per fill.
Where Chlorine Was Actually Fine
I don't want this to read as bromine-cheerleading. Chlorine worked OK for me for the months I ran it — the issue wasn't the chemistry, it was my own willingness to test daily. For someone with a different cadence, chlorine might genuinely be the better fit:
- Daily users who don't mind a daily routine — chlorine is faster and cheaper.
- People used to pool dosing — the chlorine playbook transfers more than the bromine one does.
- Budget-tight setups — chlorine is meaningfully cheaper per dose.
- Recovery between heavy bather load — chlorine acts faster on organic contamination than bromine in my (limited) experience.
How HTReminder Fits In
Whether you go bromine or chlorine, you still have to test. HTReminder is the app I built around my own testing routine — point your phone at a test strip, and it reads the colors and gives you dose recommendations for your tub size. It handles both chlorine and bromine strips. If you're switching between sanitizers and the dose math feels different, the app removes the guesswork on color reading.
Try HTReminder FreeIf You're Trying to Decide
The advice I'd give a friend: pick the system that matches your testing cadence, not the system someone else swears by. If you can test daily, chlorine is fine and cheap. If you want to test 2-3 times a week and not panic between sessions, bromine probably fits better. Run one for a full fill cycle before judging — switching back and forth doesn't tell you anything because the transitions themselves are messy.
And whatever you choose, ignore advice that doesn't account for the specific conditions in your tub. Some of what I've seen recommended (CYA, pool-style targets, mixing systems) didn't match what I was seeing in my actual water. Your tub may behave differently than mine — there's no replacement for testing and watching how your specific setup responds.