The 3 Best Swimming Pool Chlorines — How to Choose the Right One

Chlorine is the backbone of pool sanitation, but not all chlorine is the same. Trichlor, dichlor, and calcium hypochlorite (cal-hypo) each have different strengths, weaknesses, and ideal use cases. Here is how to choose the right one for your pool.

The Three Main Types of Pool Chlorine

1. Trichlor (Trichloro-S-Triazinetrione) — Best for Everyday Maintenance

Form: Slow-dissolving tablets or sticks (the familiar 3-inch tabs) Available chlorine: 90% Stabilized: Yes (contains cyanuric acid) pH effect: Lowers pH over time

Best for: Continuous, automated chlorination via a floater, skimmer basket, or automatic chlorinator.

Trichlor tablets are the most popular choice for home pools because they're convenient and long-lasting. Drop them in a floater or chlorinator and they dissolve slowly over 5–7 days, maintaining a consistent chlorine level.

Drawbacks:

• Continuously adds cyanuric acid (CYA / stabilizer) to your water. When CYA levels get too high (above 80 ppm), chlorine becomes less effective — this is called "chlorine lock." You need to partially drain and refill to bring CYA down.
• Lowers pH, so you'll need to add pH increaser regularly to compensate.

Best brands: BioLab Trichlor tablets, In The Swim 3-inch tablets, Clorox Pool and Spa

2. Calcium Hypochlorite (Cal-Hypo) — Best for Shocking

Form: Granular or tablet Available chlorine: 65–78% Stabilized: No (no cyanuric acid) pH effect: Raises pH

Best for: Shocking (superchlorinating) your pool to kill algae, bacteria, and combined chlorine.

Cal-hypo is the gold standard for pool shocking. It dissolves quickly and delivers a high dose of unstabilized chlorine that burns off combined chlorine (chloramines), kills algae, and restores water clarity.

Drawbacks:

• Pre-dissolving required — add to a bucket of water first, never broadcast directly on pool surfaces (can bleach vinyl liners and plaster)
• Raises calcium hardness over time (a concern in already-hard water)
• Raises pH; test and adjust after each shock treatment

Best brands: HTH Super Shock, Zappit 73, DryTec Calcium Hypochlorite

3. Dichlor (Sodium Dichloro-S-Triazinetrione) — Best for Spas and Vinyl Pools

Form: Fast-dissolving granules Available chlorine: 56–62% Stabilized: Yes (contains cyanuric acid) pH effect: Slightly lowers pH

Best for: Spas, hot tubs, vinyl liner pools, and any situation where you need fast-dissolving, pre-stabilized chlorine without broadcast granules.

Dichlor dissolves almost instantly, making it ideal for spas (where you want immediate sanitation) and vinyl liner pools (where undissolved granules can bleach or damage the liner).

Drawbacks:

• Like trichlor, continuously adds CYA. In a spa with high water turnover and frequent shocking, CYA can build up quickly.
• More expensive per pound of available chlorine than cal-hypo.

Best brands: In The Swim Dichlor granules, Leisure Time Renew

How to Choose: Quick Reference

The CYA Rule of Thumb

If you're using only trichlor tabs and never shocking with cal-hypo, test your CYA every 6–8 weeks. When it hits 80 ppm:

1. Stop adding trichlor
2. Partially drain the pool (25–30%)
3. Refill with fresh water
4. Test CYA before resuming trichlor

Keeping CYA in the 30–50 ppm range (or up to 70–80 ppm for outdoor pools in direct sun) keeps your chlorine working effectively.

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