Chlorine vs Bromine for a Pool

The comparison

The sunlight problem, which settles it

Chlorine breaks down in ultraviolet light quickly - HTH's stabilizer page puts the loss at up to 90% without protection. The answer is cyanuric acid, which shields chlorine from UV and is why every outdoor chlorine pool runs some.

There is no equivalent for bromine. Nothing stabilizes it, so an outdoor bromine pool loses sanitizer to sunlight continuously, all day, with no way to slow it down. On a spa - covered, indoors, or both - that does not matter. On an open pool it is the whole cost picture.

Where bromine is the better choice

• **Hot tubs and spas.** Chlorine dissipates fast in hot water; bromine is stable in it. This is the main case and it is a strong one.
• **Indoor pools.** No UV means bromine's central weakness never comes into play.
• **Pools kept covered.** Same reasoning, provided the cover is genuinely on most of the time.
• **Pools that struggle to hold pH down.** Bromine's effectiveness falls off far less at high pH than chlorine's does.
• **Swimmers sensitive to chloramines.** Bromamines are less irritating than chloramines, though bromine does linger on skin more.

Switching between them is not trivial

You cannot simply start adding bromine to a chlorine pool. The two build different reserves, and a partial conversion leaves you managing both badly.

Going from bromine to chlorine is harder still: bromide stays in the water, and every chlorine addition converts some of it back to bromine rather than sanitizing. The practical route is a substantial partial drain and refill.

The chloramine misunderstanding

The strong chlorine smell people associate with over-chlorinated pools is the opposite of what they think it is. It is chloramines - chlorine that has already combined with organic material and is spent. A pool that smells strongly of chlorine usually needs more chlorine, not less, to break the chloramines down.

Measuring combined chlorine is what tells you this, and it needs a FAS-DPD kit - see best pool test kits. Strips will not show it.

The picks

Top pick - Season-long sanitizing

In The Swim 3 Inch Stabilized Chlorine Tablets, 50 lb

Trichlor at 90% available chlorine, bought in the quantity a season actually consumes.

The published figure

Trichlor, individually wrapped, slow dissolving, 50 lb bucket

3 inch trichlor tablets are about 90% available chlorine and dissolve slowly in a floater, feeder or skimmer, which is what makes them the default residential sanitizer.

They are stabilized, so each tablet adds cyanuric acid as it dissolves. Over a season that adds up, which is why the cyanuric acid reading matters more on a tablet pool than on any other kind.

Trichlor is also acidic. Expect pH to drift down over the season and budget for alkalinity increaser.

What it does well

• Highest available chlorine of any format here at around 90%
• Slow dissolving, so a floater holds several days of sanitizer
• Individually wrapped, which slows degradation in the bucket

What it costs you

• Adds cyanuric acid continuously through the season
• Acidic, so pH drifts down and needs correcting
• Never put tablets in the skimmer if the pump cycles off — the concentrated acid sits in the line

Skip it if your cyanuric acid is already above about 50 ppm. More tablets will make chlorine progressively less effective.

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Pick 2 - Stabilizing a liquid-chlorine or salt pool

In The Swim Pool Stabilizer, 100% Cyanuric Acid, 25 lb

100% cyanuric acid at a purity that lets you compute the dose, for pools whose chlorine source does not supply its own.

The published figure

100% cyanuric acid, granular

Trichlor tablets and dichlor shock both add cyanuric acid as they dissolve. Liquid chlorine, cal-hypo and salt systems do not, so those pools need it added deliberately.

At 100% purity, 1 lb raises 10,000 gallons by 12 ppm. HTH's own stabilizer page states the useful targets: 30-50 ppm for trichlor and dichlor pools, 60-80 ppm for saltwater.

Cyanuric acid does not leave except by dilution. Adding too much is a mistake you fix by draining water, so under-dose and retest.

What it does well

• 100% purity means the arithmetic is exact
• Protects chlorine from UV loss, which HTH puts at up to 90%
• One correction lasts until you dilute the water

What it costs you

• Dissolves very slowly — 24 to 48 hours before a retest means anything
• Cannot be removed except by draining and refilling

Skip it if you sanitize with trichlor tablets. Those already add stabilizer, and your problem is more likely too much of it.

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Pick 3 - Large blooms and saltwater pools

Champion Pool Shock Liquid Chlorine 12.5%, 1 gal, 4-pack

Adds nothing but chlorine and salt, which on a big bloom or a saltwater pool is exactly the point.

The published figure

12.5% trade sodium hypochlorite, ready to use, no residue

12.5% trade means one gallon raises 10,000 gallons by 12.5 ppm. That identity makes it the easiest chemistry on this page to dose in your head, and it stops being easy the moment your pool is not 10,000 gallons — see the calculator below.

Liquid chlorine adds no calcium and no cyanuric acid. For a pool that is already hard, already stabilized, or salt-chlorinated, that is the whole argument.

It also degrades. Sodium hypochlorite loses strength in storage, faster when warm, so a jug that has sat in a hot garage since last summer is not 12.5% any more.

What it does well

• Adds no calcium and no cyanuric acid
• Dissolves instantly, no pre-mixing, no granules on the liner
• Cheapest route to a large ppm swing on a big pool

What it costs you

• Loses strength in storage, particularly in heat
• Heavy and awkward to handle at four gallons
• Raises pH, so expect to follow with acid

Skip it if you cannot store it cool and use it within a couple of months — you will be dosing an unknown percentage.

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