Chlorine tablets vs liquid chlorine: which should your pool use?

Two products, one active ingredient

Both products deliver the same working chemical into your water: free chlorine. The differences are in everything wrapped around that chlorine — how concentrated it is, how fast it releases, what else it carries into the pool, and what it does to your pH on the way.

Chlorine tablets here means trichlor — trichloroisocyanuric acid — pressed into slow-dissolve tablets. Ours are 200 g tablets, supplied in 1 kg packs and 25 kg drums. Trichlor is one of the most concentrated chlorine products made: roughly 90% available chlorine by weight, in a dry, stable tablet that erodes over days in a feeder or floater.

Liquid chlorine means sodium hypochlorite solution — chemically the same family as household bleach, but at commercial strength, typically in the range of 10–15% available chlorine when fresh. It mixes into the water instantly and leaves nothing behind except a little salt.

The honest comparison

The stabiliser catch

This is the difference that matters most over months, and the one tablet users most often discover late.

Trichlor is a chlorine molecule bonded to cyanuric acid — pool stabiliser. Every tablet releases both. The chlorine gets used up doing its job; the cyanuric acid does not. It is not consumed, it does not evaporate, and your filter does not remove it. It simply accumulates, tablet after tablet, at roughly 0.6 mg/L of cyanuric acid for every 1 mg/L of chlorine you dose.

A little cyanuric acid is useful in an outdoor pool — it shields chlorine from sunlight. A lot of it slows chlorine down: the water still shows a healthy free chlorine reading, but the chlorine kills more and more slowly. The CDC's Model Aquatic Health Code caps cyanuric acid at 100 mg/L and treats a cyanuric-acid-to-chlorine ratio above 45:1 as grounds to close a public pool. The full mechanism — and the fix, which is dilution — is covered in cyanuric acid and chlorine lock.

What each does to your pH

Neither product is pH-neutral, and the two push in opposite directions.

• Trichlor is strongly acidic — a 1% solution sits around pH 3. A pool fed on tablets tends to drift down in pH and total alkalinity, which left uncorrected takes the water into the corrosive zone: etched grout, attacked metal, stinging eyes. Tablet pools usually need periodic alkalinity top-up.
• Sodium hypochlorite is strongly alkaline in the drum, and in the pool it nudges pH upward. The drift is milder than trichlor's pull downward, but over time a liquid-fed pool tends to creep towards high pH — where chlorine loses effectiveness. Liquid-fed pools usually need periodic small acid additions.

Neither drift is a problem if you test and correct; both are a problem if you dose blind. The context for all of this is pool water balance — pH, alkalinity and hardness together.

When tablets are the right answer

• Small residential and clubhouse pools where nobody checks the plant room daily. A floater or erosion feeder keeps a steady residual between visits with no pump to fail.
• Outdoor pools in strong sun, where the stabiliser in the tablets is doing genuinely useful work — up to a point.
• Top-up and holiday cover, keeping a residual alive when the pool is unattended.
• Sites without chemical storage space — a 25 kg drum of tablets holds as much available chlorine as a much larger volume of liquid.

When liquid is the right answer

• High-turnover commercial pools — hotels, clubs, estates with heavy bather load, typically FEHD-licensed pools — where chlorine demand jumps around and you need to respond by the hour, not by the tablet.
• Pools with automatic dosing. A dosing pump and controller running on sodium hypochlorite is the standard arrangement for commercial pools, and it is what our dosing systems are built around.
• Indoor pools. There is no sunlight to protect chlorine from, so stabiliser has no benefit indoors — it only slows your chlorine. Indoor pools should generally run unstabilised.
• Any pool whose cyanuric acid is already high. Continuing tablets in that pool makes the problem worse with every dose.
• Shock dosing, where you need a large, fast, measurable addition — for example breakpoint chlorination against chloramines (see why your pool smells of chlorine).

Storage and handling: the part that is not optional

Chlorine products are hazardous chemicals, and the two types are more dangerous together than either is alone.

• Never mix them, ever. Trichlor and hypochlorite in contact — in a feeder, a bucket, a scoop, or a spill — can react violently, generating heat, toxic gases and fire. This is the standard warning on both products' safety data sheets and in the CDC's pool chemical safety guidance. Different chlorine products never share a container, a feeder or a measuring scoop.
• Keep tablets dry. Water dripping into a drum of trichlor starts a reaction in the drum. Store sealed, cool, dry and off the floor.
• Keep liquid cool and rotated. Sodium hypochlorite decays with time, heat and light. Order what you will use, use the oldest drum first, and do not stockpile — a drum bought cheap and stored for months delivers weak chlorine at full price.
• Ventilate the store, separate the acids. Chlorine products and acids (pH reducers) must be stored apart; mixed, they release chlorine gas.
• Add chemical to water, never water into concentrated chemical, and never pre-mix products in a bucket to save a trip.

Safety data sheets for our products are available on request — ask before you order if anything about handling or storage is unclear.

What we supply

We stock both, and we will tell you plainly which your pool should use — including telling you not to buy tablets if your stabiliser is already high.

• Chlorine tablets — 1 kg pack — 5 × 200 g slow-dissolve tablets, for smaller pools and top-up dosing.
• Chlorine tablets — 25 kg drum — the economical format for hotels, estates and facilities with continuous demand.
• Sodium hypochlorite (liquid chlorine) — custom volumes, suited to automatic dosing; ordered to your turnover, not stockpiled.

Prices are confirmed on enquiry. Tell us the product and quantity — or just your pool size, and we will work out what you need. We reply within one working day.

Common questions