What PoolBoss Says
Match the shock to the reading you can afford to move. Cal-hypo adds calcium, dichlor adds cyanuric acid, and liquid chlorine adds neither but loses strength in weeks. On a route, liquid is the sane default for weekly dosing, with cal-hypo held back for algae recovery and pools genuinely low on calcium.
Ask this question online and you get an answer written for somebody with one pool. Which shock is best usually means which bucket is easiest to buy at the store this weekend, and at that scale the answer barely matters. Any of the three will clear a pool once.
On a route it is a different question entirely. You are not dosing one pool once, you are dosing the same 150 pools forty or fifty times a season out of whatever you bought by the pallet. That repetition is what turns a product choice into a chemistry problem, and it shows up on the pools long before it shows up on the invoice.
At a glance
Key takeaways
- All three shocks deliver the same sanitizer, so choose on what each one leaves behind rather than on strength.
- Dichlor adds roughly 0.9 ppm of cyanuric acid per 1 ppm of free chlorine, and nothing on the truck removes cyanuric acid once it is there.
- Cal-hypo adds roughly 0.7-0.8 ppm of calcium hardness per 1 ppm of free chlorine, which only matters on fill water that is already hard.
- Liquid chlorine adds neither, which makes it the right default for weekly dosing on an established route.
- Liquid degrades fast in heat, losing around 10% of strength a month at 77F and 25-30% above 95F, so buy it in two to four week quantities instead of by the season.
- Keep cal-hypo on the truck for algae recovery, for pools genuinely under about 150 ppm calcium, and for stops you have to walk product into.
- Track calcium hardness and cyanuric acid per pool across visits, because a standardisation mistake shows up as slow drift, never as one bad reading.
What kind of pool shock should I use?
Three products are in real use on a service route, and choosing between them has nothing to do with which one kills algae hardest. All three deliver hypochlorous acid, the same active sanitizer, and all three will clear a pool. What separates them is the passenger each one brings with it.
Cal-hypo, calcium hypochlorite, runs 65-73% available chlorine and comes as granules or tablets. It is the cheapest chlorine per pound, it stores well in a hot truck, and that combination is exactly why so many routes standardise on it. Every dose also puts calcium in the water.
Dichlor is stabilised granular chlorine at 56-62% available chlorine. It dissolves quickly, it is close to pH-neutral, and it is the most convenient of the three to carry and measure. Stabilised means it carries cyanuric acid, so every dose adds some.
Liquid chlorine, sodium hypochlorite, runs 10-12.5% at professional strength against the 5-8% sold in grocery stores. It adds no calcium and no cyanuric acid, which on a route is the entire argument for it. It is heavy, it eats bed space, and it starts losing strength the day it is made.
Worth separating from the product question: whether the pool needs shocking at all. If free chlorine is present but the water is dull and the combined chlorine is climbing, that is a breakpoint problem, and knowing when a breakpoint shock is the actual fix decides the dose size. The product below decides what the dose leaves behind.
Every shock adds something besides chlorine, and on a route it accumulates
One dose of anything is harmless. The problem is the fiftieth. Dichlor adds roughly 0.9 ppm of cyanuric acid for every 1 ppm of free chlorine it delivers, which is a typical figure rather than a lab constant, but it is close enough to do the arithmetic with. Shock a pool to 10 ppm free chlorine six times across a season on dichlor and you have put something like 54 ppm of cyanuric acid into water that probably started the year around 40. That pool is now stabiliser-heavy, the chlorine you are paying for is doing less, and no product on the truck removes cyanuric acid.
That is the part operators underestimate. Almost every other reading can be corrected with a dose. Cyanuric acid cannot: bringing cyanuric acid back down means replacing water, on a pool where the customer pays for the water and, in a drought market, may not be allowed to use much of it. A stabiliser problem you dosed your way into over two seasons takes a partial drain to undo.
Cal-hypo does the same thing to calcium hardness. At 73% strength it adds roughly 0.7-0.8 ppm of calcium hardness for every 1 ppm of free chlorine, so the same six shocks put somewhere near 45 ppm of calcium into the pool across a season. On soft fill water that is genuinely fine and occasionally welcome. On Sunbelt fill water that already arrives at 300-400 ppm, it is a countdown.
Take an operator running 160 pools across Mesa and Apache Junction, Arizona, on fill water carrying 380 ppm calcium hardness. He standardised on cal-hypo years ago for the honest reason that it survives a July truck bed better than anything else. Two seasons later roughly a third of his pools read over 600 ppm calcium and are scaling on the tile, and the fix on each one is a partial drain the customer did not budget for. Nothing went wrong on any single visit. The product was correct fifty times in a row and the cumulative total was still wrong. This is the drift worth watching per pool rather than per visit, and it is why the reading history matters more than the last reading: in PoolBoss you watch calcium and cyanuric acid move across visits on the pool record, so a slow climb is visible while it is still a dosing decision and not yet a drain.
Liquid chlorine is the one product that adds neither. It contributes a small amount of salt and a temporary pH rise that largely settles as the chlorine is consumed, and nothing that builds up in a way you have to drain out. That is the whole case for it, and on a mature route it is a strong one.
The three products side by side
Read this as a standing supply decision rather than a per-pool one. The right column is the question that actually matters when you are buying for a route: what is this product good for when you use it repeatedly, not what does the label claim.
| Product | Strength | What else it adds | Shelf life in heat | Best route use |
|---|---|---|---|---|
| Cal-hypo (calcium hypochlorite) | 65-73% available chlorine, granular or tablet | Calcium hardness, roughly 0.7-0.8 ppm per 1 ppm free chlorine | Excellent, 2-3 years sealed and dry | Algae recovery, and pools genuinely low on calcium |
| Dichlor (sodium dichloroisocyanurate) | 56-62% available chlorine, granular | Cyanuric acid, roughly 0.9 ppm per 1 ppm free chlorine | Very good, 2-3 years sealed | Early-season starts on fresh fill that needs stabiliser anyway |
| Liquid chlorine (sodium hypochlorite) | 10-12.5% available chlorine, liquid | Salt, plus a temporary pH rise that largely settles | Poor, measured in weeks above 90F | The weekly workhorse on an established route |
Liquid chlorine's real cost is shelf life and truck space
The reason routes drift away from liquid is not chemistry, it is logistics, and it deserves an honest accounting rather than a shrug. Sodium hypochlorite decomposes on its own, and heat roughly doubles the rate for every 15-18F rise. Stored around 77F, a 12.5% drum typically loses in the region of 10% of its available chlorine in a month. Parked in a truck bed at 95F or above through a Phoenix August, that same month can cost 25-30%.
The practical consequences are real. Buying a season's supply at a discount in May is a false economy if a quarter of it has degraded by August, and you will not notice the loss as a bad batch. You will notice it as pools that need more product than they used to, which reads like a chemistry problem and is actually a storage problem. The tell is a route that quietly starts using more gallons for the same results in late summer.
Weight and space are the other half. A case of two 2.5-gallon jugs runs about 45 pounds and a 15-gallon carboy is around 145 pounds, against a 25-pound bucket of cal-hypo that treats far more water. On a truck that already carries acid, tabs, brushes, poles, and a vacuum, a week of liquid genuinely competes for room in a way a bucket does not.
The workable answer for most routes is a shorter restock cycle rather than a different product: buy liquid in two to four week quantities, keep it out of direct sun and off a hot metal bed, and rotate stock rather than stacking new behind old. Dose from the pool's actual volume rather than habit, since over-dosing to compensate for suspected degradation is how a route burns through the extra margin it was chasing. A free chlorine dose calculator for a known pool volume settles that faster than arithmetic on a wet clipboard.
When cal-hypo is still the right call
Standardising on liquid for weekly service is not the same as throwing cal-hypo off the truck, and a post that told you to would be wrong. There are three situations where cal-hypo is the better product, and they are common enough that most routes should carry both.
Algae recovery is the clearest. Bringing a green pool back needs a large amount of chlorine held at elevated levels for a day or more, and delivering that in liquid means carrying and pouring a lot of gallons. A few pounds of cal-hypo does the same work out of a bucket you can lift with one hand, and the calcium it contributes to a single recovery job is not the concern. The accumulation problem is about the weekly dose repeated fifty times, not about one hard shock in July.
The second is a pool that is genuinely low on calcium. Plaster and pebble surfaces need calcium hardness in roughly the 200-400 ppm band, and water below about 150 ppm is aggressive toward the surface and the grout. On those pools cal-hypo is doing two jobs at once. The judgement call is knowing which pools those are, which means reading the pool's own hardness history rather than assuming the whole route behaves like the fill water, because it is the pools at the other end of that range that develop calcium that only water changes will fix.
The third is straightforward practicality: a pool at the far end of a route where you carry product in on foot, a rooftop or courtyard pool with no vehicle access, or any stop where the difference between a 25-pound bucket and 45 pounds of jugs decides whether the visit is pleasant. Cal-hypo travels better, and that counts for something on a real day.
Dichlor keeps a narrower slot. It earns its place on a fresh fill or a newly drained pool that needs stabiliser anyway, where the cyanuric acid it carries is a feature rather than a cost. Reaching for it as a general-purpose shock on established pools is how routes end up with stabiliser problems, because the thing that makes it convenient is the same thing that accumulates.
FAQ
Frequently asked questions
Can I use liquid chlorine instead of shock, or are they different things?
They are the same thing. Shock is a dose size, not a product category. Shocking means raising free chlorine high enough to do a specific job, usually past breakpoint at roughly ten times the combined chlorine reading, and liquid chlorine reaches that number as well as any granular product does. The only real difference is concentration and therefore volume: at 12.5% you are pouring gallons where cal-hypo at 73% is scoops, so a 20,000-gallon pool taken to 10 ppm needs somewhere around 1.6 gallons of 12.5% liquid against roughly 2.3 pounds of 73% cal-hypo. Marketing has taught customers that shock is a bag with the word on it, which is worth gently correcting when it comes up, because it is the reason homeowners buy stabilised product they do not need.
How much does dichlor actually raise cyanuric acid?
Roughly 0.9 ppm of cyanuric acid for every 1 ppm of free chlorine it delivers. That figure is a working average rather than an exact constant, since it varies slightly between the dihydrate and anhydrous forms, but it is accurate enough to plan with. The practical version: a pound of dichlor in a 20,000-gallon pool adds somewhere near 3 ppm of free chlorine and about 2.7 ppm of cyanuric acid. That sounds trivial, and for one dose it is. Used as the routine weekly product it is how a pool arrives at 100 ppm of stabiliser without anyone deciding to put it there. Trichlor tabs do the same thing more slowly at about 0.6 ppm per 1 ppm, and a pool on both is accumulating from two directions at once, which is the single most common cause of stabiliser problems on a service route.
Does cal-hypo really raise calcium hardness enough to matter?
It depends entirely on the fill water, which is why the answer splits by region. Cal-hypo at 73% adds roughly 0.7-0.8 ppm of calcium hardness per 1 ppm of free chlorine. On a pool starting at 180 ppm on soft municipal water, a season of weekly cal-hypo is unremarkable and may even be useful. On fill water arriving at 350-400 ppm, which is ordinary across much of Arizona, Nevada, and inland Southern California, the same season of dosing pushes pools toward the 600 ppm range where scaling on tile and inside the heater becomes a service issue. Evaporation makes it worse, since water leaves and calcium does not, so a Sunbelt pool concentrates on its own before you add anything. The reading to watch is that pool's own trend across several months, not one number in isolation.
How long does liquid chlorine last before it loses strength?
Plan on weeks in summer heat rather than months. Sodium hypochlorite decomposes continuously, and the rate roughly doubles for every 15-18F increase in storage temperature. Around 77F, a 12.5% product typically loses in the region of 10% of its available chlorine in a month. Above 95F, which describes a truck bed through most of a Sunbelt summer, that can reach 25-30% in the same month. Sunlight accelerates it further, which is why it ships in opaque containers that are worth keeping opaque. The practical rules are simple: buy in two to four week quantities rather than seasonally, store shaded and off hot metal, rotate stock so the oldest goes out first, and treat a sudden need for more product across the whole route as a storage question before you treat it as a chemistry one.
Should I use the same shock on every pool on my route?
One default plus deliberate exceptions is the arrangement that works. Carrying three products and choosing per pool sounds rigorous and in practice produces inconsistent dosing, because the decision gets made at the pool by whoever is standing there. Pick one product as the weekly workhorse, usually liquid on an established route, and define the exceptions in advance: cal-hypo for algae recovery, cal-hypo for pools under roughly 150 ppm calcium hardness, dichlor for fresh fills that need stabiliser anyway. Write those exceptions on the pool record rather than keeping them in your head, because the moment a second tech or a fill-in covers the route, anything unwritten reverts to the default. That is also the honest reason standardisation matters more on a growing route than a solo one.
Do I need to dissolve cal-hypo before adding it?
Yes on almost every pool, and always on vinyl or fiberglass. Cal-hypo granules sink, and undissolved granules resting on a surface create a very high local chlorine concentration that bleaches plaster, stains a vinyl liner, and can etch a fiberglass gelcoat. Pre-dissolve in a clean bucket of pool water, add the product to the water rather than water to the product, stir, then pour the solution slowly around the perimeter with the pump running. Never use a bucket that has held any other chemical: cal-hypo and trichlor mixed in a container is a genuine fire and toxic-gas hazard, not a theoretical one, and it has caused real injuries. Some cal-hypo is sold in a fast-dissolving form marketed as broadcast-safe, and even then a bucket is cheap insurance on a surface you would have to pay to refinish.
Can I shock a salt pool, or does the cell handle it?
You can and often should, because a salt cell cannot produce chlorine fast enough to reach breakpoint. A residential cell generates its output gradually over hours of runtime, which is fine for holding a level and useless for the sharp spike a combined-chlorine problem or an algae bloom needs. Boosting the cell to 100% for a day is not equivalent. Liquid chlorine is the natural choice on a salt pool, since the water is already a salt solution and a modest addition changes nothing. Cal-hypo works but adds calcium to a pool that often already runs high pH, and calcium plus high pH is what scales a cell and shortens its life. Avoid stabilised products on salt pools in particular, because they typically already run cyanuric acid at 60-80 ppm by design and have no room for more.
