How much Prime for 10 gallons? (and every other size)
Seachem Prime is dosed at 5 mL — one capful — per 200 liters, 50 US gallons. That is 1 mL per 10 gallons, 0.9 mL for a 10 gallon, 0.47 mL for a 5 gallon, and 1.9 mL for a 20. The concentrated conditioners (Prime, Fritz Complete, API Tap Water Conditioner) all land in that neighborhood; the "regular strength" ones (API Stress Coat, Tetra AquaSafe, Fluval) are five times more dilute — a capful per 10 gallons. Here's the label math done for the standard tanks:
| Water treated | Prime | Fritz Complete | API Tap WC | API Stress Coat | AquaSafe Plus | Fluval WC |
|---|---|---|---|---|---|---|
| 5 gal | 0.47 mL | 0.5 mL | 0.25 mL | 2.5 mL | 2.5 mL | 2.5 mL |
| 10 gal | 0.95 mL | 1 mL | 0.5 mL | 5 mL | 5 mL | 5 mL |
| 20 gal | 1.9 mL | 2 mL | 1 mL | 10 mL | 10 mL | 10 mL |
| 29 gal | 2.7 mL | 2.9 mL | 1.5 mL | 14.5 mL | 14.5 mL | 14.5 mL |
| 40 gal | 3.8 mL | 4 mL | 2 mL | 20 mL | 20 mL | 20 mL |
| 55 gal | 5.2 mL | 5.5 mL | 2.8 mL | 27.5 mL | 27.5 mL | 27.5 mL |
| 75 gal | 7.1 mL | 7.5 mL | 3.8 mL | 37.5 mL | 37.5 mL | 37.5 mL |
From each product's label dose (sources in the calculator's product note). A cap on these bottles is 5 mL; a dropper drop is about 0.05 mL, so 0.5 mL ≈ 10 drops.
Do I dose the whole tank or just the new water?
For a water change, dose the new water — chlorine and chloramine come in with the tap water, so the amount to neutralize is set by the water you're adding, not by the tank. A 25% change on a 55 gallon tank is 13.75 gallons of new water: 1.3 mL of Prime, not the 5.2 mL that "55 gallons" would give you. Dose the whole tank's volume only when the whole tank is at risk: you filled it from the tap for the first time, you're refilling most of it, or the chlorine got in some other way (a hose left running, a municipal flush). Most people who "dose the tank" for a routine change are overdosing 4–5×. With Prime that's inside the safe margin and just wasteful; with other products it's outside the label, which is a reason to stop rather than a disaster.
Either works. If you fill from a bucket, dose the bucket, stir, pour. If you refill straight from a hose or a Python, dose the tank with the amount for the water you're about to add, just before the water goes in — dechlorinators work in seconds and there's a large excess anyway. What doesn't work is dosing after the fill is done and the fish have sat in chlorinated water for ten minutes; add it first. Chloramine (which most municipal systems now use) needs a conditioner that says it handles chloramine — all six above do — and it leaves ammonia behind that your filter, or Prime's binding, deals with over the next day.
How do I measure half a milliliter?
This is where nano tanks go wrong. A 5 gallon needs about 0.47 mL of Prime — a tenth of a cap — and the natural response is to pour "a little", which is usually a full cap, a 10× overdose. Use the drops from a pipette or a medicine dropper: about 20 drops to the mL, so a 5 gallon is 9 drops and a 10 gallon is 19. A 1 mL oral syringe from the pharmacy is better still, and costs nothing. Or, for the smallest tanks, use a regular-strength conditioner on purpose — a capful of Stress Coat per 10 gallons is far easier to halve than a tenth of a cap of Prime.
Prime: Seachem says up to 5× the normal dose is safe, and recommends that in an ammonia or nitrite emergency — so the routine "oops, a whole cap in a 10 gallon" (5.3×) is within it. Everything else: the labels say "safe if slightly overdosed" or nothing; a 2× is nobody's concern, a 10× is outside anything the maker tested. What overdosing does do at any strength: heavy doses of the sulfite-based conditioners consume oxygen as they work, which is why a big overdose in a warm, crowded, under-aerated tank is the one scenario that has hurt fish. Dose for the water you're treating, and the question doesn't come up.
Does Prime "remove" ammonia?
No — it binds it. Prime and similar products convert ammonia to a less toxic form for roughly 24–48 hours; the ammonia is still there, your test kit still reads it (usually), and it's your cycled filter that actually removes it. That makes conditioner a genuinely useful emergency measure during a cycle or a spike — buy a day, do a water change, repeat — and a poor substitute for the water change itself. If you're dosing Prime to control ammonia rather than to treat tap water, the water change calculator is the tool you want, and the tank's true volume is the number to dose against.