What to buy for your pool
The shop has thirty products and you need five. This guide says which ones, what they're for and what to read on the label, with no brands: at the same percentage of active ingredient, products are the same, and Serenpool calculates doses from exactly that percentage.
The minimum kit
| For | Product | Above-ground | In-ground |
|---|---|---|---|
| Daily disinfection | an everyday chlorine (see below which) | yes | yes |
| Shocking when the water is green | an unstabilised chlorine: liquid or calcium | yes | yes |
| Lowering pH | granular pH minus (sodium bisulphate) | yes | yes |
| Raising alkalinity | sodium bicarbonate (the supermarket kind is fine) | yes | yes |
| Testing | 6-in-1 or 7-in-1 strips, with stabiliser | yes | yes |
| Testing during a shock | drop test kit for chlorine | recommended | yes |
| Protecting chlorine from the sun | stabiliser (cyanuric acid) | only with liquid or calcium chlorine | same |
| Cloudy water with a sand filter | flocculant | no, with a cartridge | yes |
| Cloudy water with a cartridge filter | clarifier | yes | no |
Everything else, algaecide, anti-scale, winterising, "multifunction" tablets, is optional. The minimum kit for an above-ground pool costs roughly 60–90 € a season; for a 50 m³ in-ground pool 150–250 €, almost all chlorine.
1. Everyday chlorine
This is the choice that matters most. There are four families, and the label tells you which one.
| On the label | It is | Stabilised | Pros | Cons | Indicative price |
|---|---|---|---|---|---|
| Trichlor 90%, "slow chlorine", 200 g tablets | trichloroisocyanuric acid | yes, +0.6 ppm per ppm of chlorine | one tablet lasts a week in the dispenser | lowers pH; over a summer pushes stabiliser too high; no good for shocking | 10–15 €/kg |
| Dichlor 56% (or 60%), "fast chlorine", granules | sodium dichloroisocyanurate | yes, +0.9 ppm per ppm of chlorine | dissolves in minutes, handy for topping up | builds stabiliser even faster than trichlor | 10–14 €/kg |
| Calcium hypochlorite 65–70%, "shock", powder | calcium hypochlorite | no | perfect for shocking; doesn't raise stabiliser | raises hardness and pH; never in contact with trichlor | 8–12 €/kg |
| Sodium hypochlorite 14–15%, "liquid chlorine", drums | sodium hypochlorite | no | cheapest per ppm; no residue | loses strength with heat (after a summer it can be worth half); heavy to carry | 1.5–2.5 €/L |
Which to choose
- Above-ground with a cartridge filter: liquid chlorine or dichlor for topping up, plus a drum of liquid or a tub of calcium chlorine for shocking. Tablets in a floating dispenser are fine if you measure the stabiliser every month and replace some water mid-season.
- In-ground with a sand filter: trichlor tablets in the skimmer or dispenser for the routine, plus liquid or calcium chlorine for shocking. After two seasons of tablets the stabiliser is almost always above 90: measure it.
- Short on time: tablets. Want to spend less: liquid.
What to read on the label: the percentage of active chlorine (56, 65, 90, or 14–15% for liquid) and the word stabilised or the chemical name. "Fast chlorine" and "slow chlorine" are trade names: the numbers are what count.
A mistake to avoid: "multifunction" 3-in-1 or 5-in-1 tablets contain flocculant and copper sulphate. The flocculant clogs cartridge filters, the copper stains liners and hair. They only suit sand filters, with the stabiliser under control.
2. Chlorine for shocking
Green water needs chlorine raised by 10–30 points in one evening. Tablets dissolve over days and add stabiliser: they don't work. You need liquid chlorine or calcium chlorine. Always keep one pack at home, even if you use tablets: the water goes green on a Saturday.
The green water calculator works out how much. For a 15,000-litre above-ground pool it's 1 to 3 litres of liquid chlorine.
3. pH and alkalinity
- Granular pH minus (sodium bisulphate, on the label "sodium hydrogen sulphate" or "sodium bisulphate", 93–95%): the corrector you'll use most, almost every week. A 1.5 kg tub lasts an above-ground pool the whole season. Indicative price 4–7 €/kg.
- Liquid pH minus (sulphuric or hydrochloric acid): more awkward and dangerous to handle; only worth it for big pools with a dosing pump.
- pH plus (sodium carbonate): rarely needed, only with liquid or calcium chlorine that barely lower the pH. Don't buy it in advance.
- Alkalinity plus: it's sodium bicarbonate. The supermarket kind, in 1 kg packs, is the same product at a third of the price.
- Hardness plus (calcium chloride): only for concrete or tiled in-ground pools. Above-ground pools don't need it.
4. Stabiliser
Cyanuric acid protects chlorine from the sun: without it, on a July day you lose half. You only need to buy it if you use liquid or calcium chlorine, which contain none. If you use tablets or dichlor you'll have too much after a few weeks.
A 1 kg pack lasts years: in a 15 m³ above-ground pool 450 g takes you from 0 to 30 ppm. It dissolves over a week, in a sock in the skimmer. It only comes down by replacing water: the guide on high stabiliser explains when.
5. What to test with
| Tool | Indicative price | What it measures | Good for |
|---|---|---|---|
| 3-in-1 strips | 5–8 € | chlorine, pH, alkalinity | the routine, but without stabiliser you can't tell why chlorine isn't working |
| 6-in-1 or 7-in-1 strips | 8–12 € | also stabiliser, hardness, total chlorine | the right choice: with the stabiliser the app works out the chlorine to keep |
| Drop test kit (DPD or OTO) | 10–20 € | chlorine, pH | shocking: strips can't read chlorine above 10 |
| Photometer | 100–250 € | everything, accurately | large in-ground pools and pool technicians |
Strips expire: buy a fresh pack at the start of the season and keep it closed and dry.
6. Extras: when yes, when no
- Algaecide (quaternary ammonium or polyquat): prevents, doesn't cure. Useful after clearing green water and in the hottest weeks, a small weekly dose. Too much makes foam.
- Flocculant: only with a sand filter. It settles dirt on the floor, then you vacuum to waste. With a cartridge it clogs it in an hour.
- Clarifier: the cartridge filter version. It helps the filter catch fine particles without clogging.
- Anti-scale or sequestrant: one dose at opening, if your tap water is hard.
- Winteriser: only for in-ground pools that stay full over winter. Above-ground pools come down.
- Active oxygen: the "chlorine-free" alternative. It costs 3–4 times as much, oxidises but doesn't disinfect for long: in an above-ground pool in the sun the water goes green anyway.
7. Safety in storage
- Trichlor and calcium chlorine never in the same pack, the same dispenser or touching: the mix can catch fire.
- Chlorine and pH minus on different shelves: together they give off toxic gas.
- Packs closed, in the shade, kept apart, away from children. Liquid chlorine somewhere cool: every month in the heat it loses strength.
- The product goes into the water, never water onto the product. Granules are dissolved in a bucket first.
How Serenpool uses this list
In the app, under More → What to buy, you'll find the same guide tailored to your pool: the kit for an above-ground pool with a cartridge isn't the one for an in-ground pool with sand. When you choose your products in Settings, the app works out every dose from their percentage. With Pro, on the Savings page, you enter what you pay and it tells you which chlorine really costs you least.
Prices are indicative ranges and change with pack size and season. Doses are calculated from the percentages stated on labels: always check your product's label.