Water conditioner (dechlorinator) is one of the least-glamorous purchases in the hobby and one of the most essential. UK tap water contains chlorine or chloramine — both toxic to fish, both instantly kill beneficial filter bacteria. Add tap water untreated and you either kill fish outright or crash your tank’s biological cycle.
This guide covers the water conditioners actually available in the UK in 2026, what each does, and why one product has become the de-facto default among serious aquarists.
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- Water conditioner (dechlorinator) neutralises the chlorine and chloramine in UK tap water that would otherwise kill fish and beneficial bacteria.
- Seachem Prime is the best value — ultra-concentrated, treats chloramine, and detoxifies ammonia during emergencies.
- Dose every time tap water enters the tank, including top-ups.
- At 5× dose, Prime buys time during an ammonia spike by temporarily detoxifying it.
Our top picks at a glance
- Best overall: Seachem Prime — concentrated (5ml treats 200L), detoxifies ammonia and nitrite too, £10 lasts a year.
- Best budget: API Tap Water Conditioner — basic dechlorinator, cheaper by volume.
- Best for shrimp tanks: Seachem Prime — copper-free.
- Best premium: Nutrafin AquaPlus — adds beneficial slime-coat additives.
- Best for large tanks: Seachem Safe (powder form) — 100g treats 4,000L, hugely economical.
What water conditioner actually does
The good products do three things:
1. Neutralise chlorine. UK water companies chlorinate tap water at ~1 mg/L. Chlorine dissipates from open buckets within 24 hours but is instantly fatal to fish. Dechlorinator (sodium thiosulphate) neutralises chlorine chemically within seconds.
2. Neutralise chloramine. Newer water treatment technique — chloramine is chlorine + ammonia bound together. Doesn’t dissipate from buckets. Requires stronger dechlorinators that break the chloramine bond AND detoxify the resulting ammonia. Basic dechlorinators may not fully treat chloramine. Check your water company’s website for which they use.
3. Bind heavy metals. UK tap water can contain trace copper (from copper pipes), iron, zinc. All toxic to shrimp and moderately toxic to fish. Good conditioners chelate (bind) heavy metals harmlessly.
Bonus features some products add:
- Detoxification of ammonia and nitrite (Prime specifically)
- Slime-coat boosters (aloe vera, artificial polymers)
- Herbal additives (Melafix-style ingredients)
The detailed picks
Best overall — Seachem Prime
- Where to buy: Seachem Prime 100ml · Seachem Prime 500ml
- Concentration: ultra-concentrated. 5ml treats 200L.
- Price: £10 (100ml) — £22 (500ml). Lasts 12–24 months.
The standard among serious aquarists globally. Genuinely dose-effective — you use half the amount of most competitors. Detoxifies chloramine completely (breaks the chlorine-ammonia bond AND detoxifies the ammonia for 24-48 hours). Chelates heavy metals. Safe for freshwater, marine, brackish.
Critically for aquarists in trouble: at 5× dose during ammonia spikes, Prime detoxifies up to 1 ppm of ammonia and nitrite for 48 hours — buys you time during emergencies. See our how to save a dying fish guide.
Overkill for a small nano tank; correct choice for anyone with more than one tank.
Best budget — API Tap Water Conditioner
- Where to buy: API Tap Water Conditioner 30ml
- Concentration: basic. 5ml treats 40L.
- Price: £4–£8. Lasts 3–6 months.
Simple dechlorinator, doesn’t add extras. Fine for regular water changes; less effective in emergencies. Cheaper per bottle but not cheaper per litre-treated when you calculate correctly.
Fine choice for anyone who wants a straightforward dechlorinator without the complexity.
Best premium — Nutrafin AquaPlus
- Where to buy: Nutrafin AquaPlus 120ml
- Concentration: medium. 5ml treats 40L.
- Price: £8–£12.
Dechlorinates + neutralises chloramine + adds artificial slime-coat additives. Marketed as reducing stress from handling.
Debatably useful — the slime-coat additives are marginal in effect. Choose over Prime if you specifically want the slime-coat feature; otherwise Prime is more concentrated and cheaper per treatment.
Best for large tanks — Seachem Safe (powder)
- Where to buy: Seachem Safe 100g
- Concentration: insanely concentrated. 1g treats 40L.
- Price: £20–£25. Lasts 5+ years for a small tank.
The concentrated powder form of Prime. Absurdly cost-effective if you’re treating large volumes weekly (200L+ tanks, or multiple tanks).
Downsides: dosing very small tanks is difficult (need a precise 0.1g scale). Not for beginners.
Marine-specific — Seachem Prime works, or use marine-specific
For marine (saltwater) tanks, Seachem Prime works fine. Alternatively:
- Seachem PrimeReef — marine-specific formulation
See our marine tank setup guide.
Cost comparison (2026 UK prices)
| Product | Bottle price | ml/kg | Treats | Cost per 200L treated |
|---|---|---|---|---|
| Seachem Prime 500ml | £22 | — | 20,000L | £0.22 |
| Seachem Safe 100g | £22 | 100g | 4,000L | £0.11 |
| Nutrafin AquaPlus 120ml | £11 | — | 960L | £2.30 |
| API Tap Water Conditioner 30ml | £6 | — | 240L | £5.00 |
Prime and Safe are 10-20× cheaper per treatment than API and Nutrafin. This is why the aquarium community has largely consolidated on Prime as the default.
When to use conditioner
Every time water enters the tank from tap:
- Weekly water changes — dose the bucket, wait 30 seconds, pour into tank
- Emergency water changes — always
- Top-up water for evaporation — always
- Setting up a new tank — dose the entire tank volume
Not needed:
- Water changes with previously-treated water
- RO water (already free of chlorine)
- Bottled spring water (already free of chlorine, though check the label)
The proper dosing protocol
- Fill bucket with tap water at matching temperature
- Add water conditioner per the label (5ml Prime per 200L; 10ml API Tap Water Conditioner per 80L)
- Stir briefly, wait 30 seconds
- Pour slowly into the tank
Do not dose the entire tank at once during regular water changes — only dose the volume of new water being added.
Emergency exception: if you’ve somehow added untreated tap water to a tank, dose the whole tank volume immediately. Fish gasping is 15–60 minutes from death; act fast.
What to avoid
- “Aquarium safe” household products — dishwasher salt, kitchen products marketed for aquariums. Skip.
- Very cheap unbranded dechlorinators — questionable chelation, unreliable chloramine handling.
- Skipping dechlorinator “just this once” — the one time you don’t dose is the time your tap water has an unusually high chlorine dose from routine water-company maintenance.
The pattern for UK aquarists
Buy Seachem Prime 500ml. Costs £22 up front, lasts 12–24 months of weekly water changes, handles both chlorine and chloramine, chelates heavy metals, detoxifies emergency ammonia spikes, works for freshwater and marine.
That single product replaces the four or five specialist products less-informed aquarists buy separately.
Bottom line
For 95% of UK freshwater aquarists: Seachem Prime. Buy 500ml, use for a year, refill.
On a strict budget: API Tap Water Conditioner works. Just be aware it costs more per treatment long-term.
Never skip. Chlorine + chloramine + heavy metals are the invisible killers of new fish. £10 for a bottle beats £30 in replacement fish and £50 in restart costs.
Frequently asked questions
What is the best aquarium water conditioner?
Seachem Prime is the standard choice — it is highly concentrated (5 ml treats 200 litres), neutralises both chlorine and chloramine, and detoxifies ammonia and nitrite in emergencies.
Do I need to use a water conditioner every water change?
Yes, every time tap water enters the tank. UK tap water contains chlorine or chloramine, which is fatal to fish and kills the beneficial bacteria in your filter.
Can I use too much water conditioner?
Seachem Prime is safe at up to 5× the normal dose, which is actually recommended during ammonia emergencies. Follow the label for routine water changes.
Related: how to cycle a fish tank, how to clean a fish tank (uses water conditioner every week), how to lower nitrate in fish tank, how to save a dying fish (Prime helps here).
— Jaqline Smalling, FishForever
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