Cal-Hypo vs Dichlor vs Trichlor

The comparison

Cal-hypo: the shock

Calcium hypochlorite is the default emergency chlorine because it is strong, cheap per ppm and unstabilized. That last property is the important one: you can use it as many times as a season demands without pushing cyanuric acid up.

What it does add is calcium hardness. On a pool with soft fill water that is a small bonus. On one already above 400 ppm calcium it is a step toward scale, and liquid chlorine is the better option.

It also raises pH, which is worth planning for: HTH's own pH Down page advises waiting four to six hours after shocking before adding acid, because dosing the two together is how people overshoot.

Dichlor: convenient, and it accumulates

Sodium dichloroisocyanurate dissolves fast enough to broadcast directly, which is genuinely useful on a vinyl liner where undissolved cal-hypo granules bleach the pattern. It is also pH neutral, so it does not need a correction behind it.

Trichlor: the sanitizer people mistake for shock

Trichlor is around 90% available chlorine, the highest of any of these, and it is still the wrong product for a bloom. It is compressed into slow-dissolving tablets specifically so it releases over days, which is exactly what you do not want in an emergency.

It is also strongly acidic. A season of tablets pulls pH and alkalinity down steadily, which is why tablet-fed pools need alkalinity increaser as a matter of routine rather than as a correction.

Liquid chlorine: the one that adds nothing

Sodium hypochlorite adds salt and water. For a pool that is already hard, already stabilized, or salt-chlorinated, that is the entire argument and it is a strong one. The costs are storage life and handling weight - see liquid chlorine vs shock for the direct comparison.

A sensible season

• **Sanitizing:** trichlor tablets in a floater or feeder, sized at roughly one tablet per 10,000 gallons per week - HTH's own figure, which it says holds free chlorine at 1 to 4 ppm.
• **Weekly shock:** cal-hypo, so the routine dose is not also a stabilizer dose.
• **Emergency shock:** cal-hypo, weighed, or liquid chlorine on a hard-water pool.
• **Mid-season correction:** test cyanuric acid. If tablets have pushed it past 50 ppm, switch the sanitizer rather than shocking harder.

The picks

Top pick - Clearing an algae bloom

In The Swim Pool Shock, 68% Cal-Hypo Granular, 25 lb

The highest available-chlorine percentage in common retail cal-hypo, sold loose so you dose by weight instead of by packet.

The published figure

68% available chlorine, calcium hypochlorite, sold loose by the pound

68% available chlorine means 1 lb raises 10,000 gallons by about 8 ppm. A green pool typically needs 10 to 30 ppm depending on how far the bloom has gone, which is 1.2 to 3.7 lb per 10,000 gallons.

Loose granular is the format that lets you weigh a dose. Pre-measured 1 lb packets are convenient and fine for weekly maintenance, but they force you to round a shock dose to the nearest pound, which on a small pool is a 30% error.

Cal-hypo adds calcium hardness and no cyanuric acid. That is the right trade for a bloom and the wrong one for a pool that already sits above 400 ppm calcium.

What it does well

• 68% is the top of the common retail range, so you carry and store less product per ppm
• Loose granular lets you weigh an exact dose rather than round to a packet
• Adds no cyanuric acid, so shocking repeatedly does not drive you into chlorine lock

What it costs you

• Adds calcium hardness with every dose
• Must be pre-dissolved in a bucket for vinyl liners, which the label states
• A 25 lb bucket is more than a small pool will use in a season

Skip it if your fill water is already hard, or you own a small above-ground pool that will never work through 25 lb before the bucket cakes.

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Pick 2 - Vinyl liners and fiberglass

In The Swim Sodium Dichlor Chlorine Shock Granules, 25 lb

The liner-safe shock in a format you can actually weigh, if you are prepared to watch cyanuric acid climb.

The published figure

Sodium dichloroisocyanurate, fast dissolving, sold loose

Dichlor dissolves fast enough to broadcast directly, which is the practical argument for it on a vinyl liner where undissolved cal-hypo granules bleach the pattern.

Buying it loose rather than in packets is the same argument as the cal-hypo above: an exact dose beats a rounded one.

Keep a cyanuric acid test to hand if this is your regular shock. The stabilizer it adds is the reason it needs watching.

What it does well

• Safe to broadcast on vinyl and fiberglass without pre-dissolving
• Loose format allows a weighed dose
• pH neutral

What it costs you

• Adds cyanuric acid on every dose
• 56% available chlorine means more product per ppm than cal-hypo

Skip it if you shock weekly. At that cadence stabilizer accumulation is not a footnote, it is the outcome.

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Pick 3 - 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 4 - 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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