Dose by new water added, not tank size
The math is simple. Dechlorinator binds chlorine and chloramine in whatever water it touches. You only need to bind the chlorine in the NEW water — your tank water is already dechlorinated from the last change. Dosing for the whole tank is wasteful and, on large-balance products like Prime, expensive.
Example: a 50-gallon tank doing a 25% water change. New water volume is 12.5 gallons. Prime dose: 1.25 mL (a quarter of a 5 mL capful). The whole-tank dose would be 5 mL — a full capful. Over a year of weekly water changes that's the difference between one bottle and four.
The trap: most online calculators dose for the wrong volume
Almost every dechlorinator calculator at the top of search asks for tank size, then doses the whole tank. That gives 4x the correct dose on a 25% water change, 2x on a 50% change, 10x on a routine 10% change. The math isn't hard; the calculators are just built that way because asking for tank size is the easier UX question.
Overdosing dechlorinator isn't acutely dangerous at consumer concentrations — Prime is documented safe up to 5x — so the misdose doesn't kill fish, just empties bottles. Worth getting right anyway.
Per-product dose rates (sourced from manufacturer spec sheets)
- Seachem Prime — 1 mL per 10 US gal (5 mL = 1 capful per 50 gal). Concentrate. Binds ammonia/nitrite for 24-48 hours. Up to 5x dose documented for emergencies.
- API Stress Coat — 5 mL per 10 US gal. Adds aloe vera for slime-coat support. Standard concentration.
- Tetra AquaSafe Plus — 5 mL per 10 US gal. Standard concentration; treats chlorine + chloramine.
- Fluval Aqua Plus — 5 mL per 10 US gal. Treats chlorine + chloramine.
- Kordon NovAqua+ — 5 mL per 10 US gal. Adds electrolytes + colloidal aloe.
Always cross-check the bottle label — manufacturers occasionally adjust formulations. The calculator above uses the current published dose rates as of April 2026.
When to use each product
Practical decision rules:
- Default pick — Seachem Prime. Cheapest per gallon treated, smallest bottle to store, binds ammonia/nitrite during a cycling event or accident. Has the documented 5x emergency dose other products lack.
- Picking by aloe-coat additive — API Stress Coat. If you specifically want the slime-coat support after a fish shipment / handling event. Marginal benefit on routine changes.
- Already on it — whatever you have. All five products correctly neutralize chlorine and chloramine at their stated doses. Switching brands has minimal upside on a healthy tank.
What to avoid: very old single-purpose "chlorine remover" formulations that don't handle chloramine. If your municipal water uses chloramine (~70% of US utilities), older sodium-thiosulfate-only products will leave the released ammonia in your water. The five products above all handle both.
FAQ
Why is your calculator asking for new water added, not tank size?
Because dechlorinator only needs to neutralize chlorine in the water you're ADDING. A 25% water change on a 50-gallon tank introduces 12.5 gallons of new water; that's what needs treating. Most online calculators ask for tank size and dose for the full 50 gallons — that's 4x the correct dose. Overdosing dechlorinator isn't acutely toxic at small multiples (Prime is documented safe up to 5x), but you're wasting product and money. Get the math right.
Should I dose the bucket or the tank when doing a water change?
Either works for routine dechlorination. Dose the bucket and stir, then add to the tank — that's the conservative path because chlorine never enters the tank. Dose the tank and add water — also fine on a tank with established bacterial colonies, because a tank's volume buffers the chlorine until it's bound (typically within seconds for free chlorine, longer for chloramine). On a freshly cycled tank with delicate fish, dose the bucket. On an established 50+ gallon tank with hardy fish, dose the tank for convenience.
What's the difference between chlorine and chloramine dosing?
Free chlorine is what older municipal systems use; it dissipates from water in 24-48 hours of standing. Chloramine is chlorine bonded to ammonia, used by ~70% of US water utilities; it persists for weeks and the chlorine + ammonia have to be neutralized separately. Most modern dechlorinators (Prime, Stress Coat Plus, Fluval Aqua Plus, Tetra AquaSafe Plus) handle both at the same dose rate. Older single-purpose chlorine removers do NOT handle chloramine — read the label.
What's the emergency 5x dose for, and is it safe?
Seachem Prime is documented to bind ammonia and nitrite up to 1 ppm at 1x dose, with the manufacturer stating up to 5x dose for emergencies (acute ammonia spike during cycling, after a power outage, etc.). The 5x dose is documented as safe by Seachem on their dose page; it's not a routine dose. Don't use it weekly — it's a stopgap for an acute spike while you do a water change. After 24-48 hours the bound ammonia/nitrite re-releases for the bacterial colony to consume; if your colony isn't established, you have to keep dosing OR keep doing water changes. Other dechlorinators do not have this documented emergency capability.
Why does Prime dose so much less than API Stress Coat?
Prime is a concentrate (1 mL per 10 gal) while Stress Coat is a standard formula (5 mL per 10 gal). Prime costs more per bottle but lasts 5x as long; Stress Coat is cheaper per bottle but you go through it faster. On a per-gallon-treated basis, Prime is usually 20-40% cheaper than Stress Coat at typical retail prices, especially in the larger 500 mL+ sizes. The functional difference: Prime additionally binds ammonia and nitrite for 24-48 hours; Stress Coat adds aloe vera (claimed slime-coat support) which Prime does not. Pick by what your tank actually needs.
Will overdosing dechlorinator hurt my fish?
Routine 2-3x overdose: no documented harm at consumer-product concentrations. Prime is documented safe at 5x; most other products tolerate 2-3x without obvious effect. The risk at higher overdoses (10x+) is sulfite-based products (older formulations) consuming oxygen as they bind chlorine, lowering dissolved oxygen — which matters in a low-flow tank with high stocking. Not all dechlorinators use sulfites (Prime uses a different chemistry; manufacturer doesn't disclose specifics). Avoid extreme overdose, but don't panic if you accidentally pour a 2x amount.
What if I'm using well water? Do I need dechlorinator?
Well water doesn't have chlorine or chloramine — utilities add those. If your water comes from a private well, you don't need dechlorinator for chlorine. You may still want a water conditioner for other reasons: heavy-metal binding (Prime claims this), aloe vera slime-coat additives (Stress Coat), or just to neutralize trace chemistry from agricultural runoff. Test the well water with a chlorine kit first — if zero chlorine, save the money.
Related
- Water change calculator →
- Aquarium volume calculator →
- How to cycle a new aquarium →
- Cloudy water — causes and fixes →
Math sourced from manufacturer spec sheets (Tier 2 per FTM editorial policy). For sick fish or tank emergencies, talk to an aquatic veterinarian or a qualified local aquarium professional — this is calculator math, not veterinary advice. See methodology for the full sourcing tier list.