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?
Use the volume your product’s label specifies for the method you use. When treating replacement water in a separate container, dose for that new water. A 25% change on a 55 US gallon tank prepares 13.75 gallons: 1.3 mL of Prime at the metric label rate used here. Seachem’s directions say to base the dose on aquarium volume when adding Prime directly to the aquarium: 5.2 mL for this system. Select “The whole tank” above for that method. Other products can have different instructions. Put the chosen amounts on your maintenance checklist.
Seachem prefers treating new water first. Measure the replacement volume, add the label dose and follow its mixing instructions before introducing it. A direct refill needs a product that allows that method, its specified dose basis and its timing instructions; do not assume the bucket dose applies. Chloraminated water needs the appropriate chloramine rate on the exact bottle. Do not leave fish exposed to untreated tap water while waiting until the refill is finished.
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.