Pool shock, bromine, floc and algaecide — not the same tub

Spa-shop aisles sell ‘shock’, ‘clarifier’, ‘algaecide’ and ‘spa bromine’ as if they were flavours of the same powder. They are not. Free chlorine, combined chlorine, pH, alkalinity, hardness and (outdoors) cyanuric acid tell you which molecule to weigh.

The basics guide covers soda ash, dry acid, TCCA and CYA. This page is the rest of the plant room: oxidisers, dichlor vs cal-hypo, bromine, floc, metals and the reducer you use when you overshot a shock.

Cloudy after a party usually wants an oxidising shock first — dichlor granules or cal-hypo, not a bottle labelled ‘clarifier’. Dead organics then need to leave the water: aluminium sulphate floc drops a blanket you vacuum to waste. A polymer clarifier (polyDADMAC, CAS 26062-79-3) is a different tool for filter-assisted polish — see the Blue Horizons Ultimate Water Clarifier teardown. It is not alum.

If the filter is dirty or the pH is wild, more powder will not help. Test, clean the filter, then pick one chemistry.

Fi-Clor Superfast-style granules are calcium hypochlorite (CAS 7778-54-3) — an unstabilised oxidiser. About 10 g per cubic metre is a typical ~7 ppm shock; pre-dissolve, never mix dry with acid or TCCA. Cal-hypo raises calcium hardness. Indoor pools often prefer it because it does not pile on CYA.

Clearwater chlorine granules are dichlor — sodium dichloroisocyanurate dihydrate (CAS 51580-86-0). About 2 g per 1,000 L raises 1 ppm; 10 g/m³ is a ~5 ppm shock. Fast dissolve, near pH-neutral, and it adds cyanuric acid. Do not stack dichlor on TCCA if CYA is already high.

Clearwater ‘non-chlorine shock’ is MPS triple salt (CAS 70693-62-8). Typical spa dose is on the order of 12 g per cubic metre. It burns off organics without adding chlorine — and without leaving a sanitiser residual. Some test strips read MPS as chlorine; wait and retest.

Hot-tub bromine tablets are 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH, CAS 16079-88-2). They go in a floater, not loose on acrylic. Target 3–5 ppm bromine in hot water. A 5 kg trade tub is the same 20 g pucks as the 1 kg spa tub.

Do not mix concentrated bromine with cal-hypo or dichlor in a bucket. Convert or drain on purpose — do not invent a dual-sanitiser cocktail.

HTH / Fi-Clor granular floc is aluminium sulphate (CAS 10043-01-3). A typical drop-out dose is about 25 g per cubic metre after a shock, with pH near 7.2–7.6. Stop the pump, settle overnight, vacuum to waste. Not a cartridge-filter magic powder and not polyDADMAC. The polymer clarifier teardown is the filter-on polish; alum is the vacuum-to-waste drop.

Scale and stain bottles (Lo-Chlor HEDP, CAS 2809-21-4) sequester calcium and metals — they do not drop a cloud. Hardness Plus is calcium chloride (CAS 10043-52-4) when the water is too soft. Different jobs; do not stack them because the labels all say ‘clear’.

Lo-Chlor copper algaecide is copper sulphate pentahydrate (CAS 7758-98-7). Hold about 0.5 ppm copper; overdose stains plaster and hair. Pre-dissolve; never dump crystals on a vinyl liner; never mix concentrated copper with concentrated hypochlorite in a bucket.

Chlorine / bromine reducer is sodium thiosulphate (CAS 7772-98-7) — photographer’s hypo. On the order of 3 g per cubic metre per ppm you want to remove, then retest. Too much and you have no sanitiser left. Never mix the dry powder with an oxidiser in a bucket.

Add with circulation running. Never combine concentrated acids, chlorines, bromine, copper or thiosulphate in the same bucket. The teardowns give pack maths for a labelled salt — they are not a substitute for a test kit or a plant-room manual.