Walk into any pool supply store and you'll see shock products labeled "Super Shock," "Power Powder," "Rapid Shock," and dozens of other names. Underneath the marketing, there are really two main shock chemistries: calcium hypochlorite (cal-hypo) and sodium dichloro-s-triazinetrione (dichlor). Each has a distinct chemistry profile, side effects, and ideal use case. Choosing wrong means either driving up calcium hardness or steadily accumulating CYA — both of which create problems later.
| Property | Cal-Hypo (68%) | Dichlor (56%) |
|---|---|---|
| Full name | Calcium hypochlorite | Sodium dichloro-s-triazinetrione |
| Available chlorine | 65–78% | 56–62% |
| Adds CYA? | No | Yes (~9 ppm per 10 ppm Cl₂) |
| Adds calcium? | Yes (~7 ppm per lb per 10K gal) | No |
| pH in solution | ~11.5 (raises pH) | ~7.0 (neutral) |
| Dissolution speed | Medium — pre-dissolve recommended | Fast — dissolves quickly |
| Compatible with SWG? | Use with caution (check CH level) | Yes, but watch CYA accumulation |
| Typical cost per lb | $1.50–2.50 | $2.00–3.50 |
| Shelf life | 2–3 years stored dry and cool | 3–5 years stored properly |
Calcium hypochlorite at 65–78% is the standard professional shock choice. At 68% available chlorine, it is the highest-concentration solid chlorine product commonly available — only chlorine gas is stronger. One pound added to 10,000 gallons raises free chlorine by approximately 7–8 ppm.
Every pound of cal-hypo also releases calcium into the water — roughly 5–8 ppm of calcium hardness per pound per 10,000 gallons. For a pool already at 350 ppm CH, weekly cal-hypo shocking over a 20-week season can push CH from 350 to 500+ ppm. That creates a scaling risk and potentially an expensive drain situation.
Cal-hypo also raises pH significantly — the product itself has a pH of approximately 11.5. Add a 1-pound dose to a 10,000-gallon pool and expect pH to rise 0.3–0.5 temporarily. With proper alkalinity buffering, it comes back down within 24 hours. Without proper buffering, pH can stay elevated for days.
Dichlor granules are 56–62% available chlorine with a neutral pH of approximately 7.0. They dissolve fast, don't require pre-dissolving, and won't raise pH appreciably. Sprinkle them directly into the pool while walking the perimeter.
Dichlor is a chlorinated isocyanurate — the same chemical family as trichlor tablets. This means it contains cyanuric acid bound to the chlorine molecule. When it dissolves, it releases both chlorine and CYA. Approximately 57% of dichlor's weight is cyanuric acid. For every 10 ppm of chlorine added, dichlor also adds about 9 ppm of CYA.
Many pool professionals prefer liquid chlorine (sodium hypochlorite, 12.5%) for all shocking duties — including green pool recovery. Liquid chlorine adds zero CYA, zero calcium, and gives precise control over dose. The trade-off is bulk — you need much more liquid than granular shock to achieve the same chlorine level. A SLAM treatment for a 15,000-gallon pool at 50 ppm CYA requires approximately 3–4 gallons of 12.5% liquid chlorine just to reach shock level.
For a service truck serving residential pools, the practical recommendation is:
PoolLens logs which shock product you used, at what dose, on what date for every account. Know which pools are approaching CH or CYA limits from shock accumulation — before it becomes a drain situation.
Open PoolLens Free →Cal-hypo (calcium hypochlorite) is 65–78% available chlorine, raises calcium hardness, has no CYA, and a pH around 11. Dichlor is 56–60% available chlorine, adds approximately 9 ppm CYA per 10 ppm of chlorine added, and has a pH around 7.0. Cal-hypo is better for pools with low calcium; dichlor is useful for pools with low CYA that need a quick boost.
Never mix cal-hypo and dichlor directly. When combined in concentrated form, they react violently and can cause fire, explosion, and release of chlorine gas. These two products must be added separately — never pre-mix and never add to the same skimmer within the same hour.
Yes. Dichlor is approximately 57% cyanuric acid by molecular weight. Every 10 ppm of chlorine added via dichlor also adds approximately 9 ppm of CYA. Frequent dichlor shocking can rapidly increase CYA to problematic levels.
Yes. Calcium hypochlorite releases calcium into the water as it dissolves. Each pound of 68% cal-hypo added to 10,000 gallons raises calcium hardness by approximately 5–8 ppm. In already-hard water pools, regular cal-hypo shocking can push CH into scaling territory.