Superchlorination: What It Means and How to Do It Right
Superchlorination means temporarily raising your pool's free chlorine to a much higher-than-normal level to oxidize contaminants, kill algae and bacteria, and clear up water problems. In everyday use, "superchlorination" and "shocking" mean the same thing: a high-dose chlorine treatment. The key to doing it right is knowing how high to go and why, which depends heavily on your cyanuric acid (CYA) level.
Superchlorination, shock, and breakpoint
These words get used loosely, so here's how they actually relate:
- Superchlorination / shocking: raising free chlorine well above normal to oxidize contaminants and kill pathogens.
- Breakpoint chlorination: a specific goal of superchlorination. You add enough chlorine to oxidize combined chlorine (chloramines) so it drops back down. If you don't add enough to reach breakpoint, chloramines tend to persist, and can keep forming, so the smell and irritation don't go away.
That "chlorine smell" and eye irritation are usually signs of chloramines and poor circulation, not too much chlorine. (Very high chlorine or out-of-range pH can also irritate, so balance matters too.)
Why CYA changes everything
Here's the part pool stores rarely explain: how much chlorine you need to shock depends heavily on your cyanuric acid (CYA) level. CYA buffers chlorine, so a pool with 80 ppm CYA needs far more free chlorine to reach an effective shock level than a pool with 30 ppm CYA. That's why a generic "add one bag per 10,000 gallons" instruction is unreliable. It ignores CYA, your starting chlorine, and the product's strength.
Pro tip: Modern DIY pool care (popularized by TroubleFreePool) ties shock level directly to CYA rather than pool volume alone. The higher your CYA, the higher the free chlorine target. That's also why letting CYA climb too high makes shocking expensive and difficult.
SLAM: the method for a green or algae pool
When you're clearing an actual algae bloom, a single dose of shock isn't enough. The modern approach is called SLAM (Shock Level And Maintain): raise free chlorine to the correct shock level for your CYA, then hold it there, retesting and re-dosing throughout the day, until all three of these are true:
- The water is clear;
- Combined chlorine reads 0.5 ppm or less;
- An overnight chlorine loss test shows a drop of 1.0 ppm or less (meaning nothing is left consuming chlorine overnight).
The mistake most people make is stopping after one dose or one day. SLAM is about maintaining the level, not hitting it once. (See Green Pool for the full triage.)
When to superchlorinate
- Combined chlorine around 0.5 ppm or higher (that "chlorine smell," burning eyes).
- Algae: green, cloudy, or the start of a bloom.
- After heavy use or storms (big bather loads or runoff).
- Contamination incidents: for a fecal or diarrheal incident, follow a CDC-style free-chlorine-and-time protocol (which accounts for CYA), not just a one-time shock dose.
- Water that looks dull or won't clear despite normal chlorine.
What to shock with
- Liquid chlorine (sodium hypochlorite) or cal-hypo are the workhorses for raising free chlorine to shock levels. Liquid adds no CYA or calcium; cal-hypo adds calcium.
- Avoid stabilized shock (dichlor/trichlor) for routine shocking. They add CYA every time, driving your CYA up and making future shocking harder.
- Non-chlorine shock (potassium monopersulfate, "MPS") is an oxidizer that clears organic bather waste and lets you swim sooner, but it is not a sanitizer and is not a reliable way to kill algae, so it won't fix a green pool. Note: MPS can cause false combined-chlorine readings on some test kits unless you use an MPS-neutralizing reagent.
Doing it safely
When is it safe to swim again? It's generally safe to swim once your free chlorine is at or below the recommended maximum for your CYA level (many DIY methods allow swimming right up to SLAM level), your pH is in range, and the water is clear enough to see the bottom. Some people use "10 ppm or less" as a rough comfort rule, but the CYA-based rule is more reliable. Always test before anyone gets in. Don't rely on time alone.
Spas and hot tubs
Hot tubs are shocked differently. Smaller water volume and high temperature mean doses are much smaller, and many spa owners use non-chlorine (MPS) shock for routine oxidation between sanitizer doses. See Spa Shock for spa-specific guidance.