Quarantine Tank Size Calculator
Calculate quarantine tank size in gallons from fish size and count.
Returns minimum volume, water change schedule, and equipment for safe fish isolation.
A quarantine tank (QT) is a separate aquarium used to isolate new fish before introducing them to your main display tank. This practice prevents diseases like ich, velvet, and bacterial infections from wiping out an established community.
Quarantine Tank Sizing Formula
Minimum QT Volume (gallons) = (Number of Fish × Fish Adult Size Factor) + Base Volume
Where:
- Base Volume: 10 gallons minimum for any quarantine setup (enough for a heater, sponge filter, and hiding spots)
- Fish Adult Size Factor: gallons per fish based on adult body length
| Fish Adult Length | Gallons per Fish | Examples |
|---|---|---|
| Under 2" (small) | 1.5 gal | Neon tetras, endlers, ember tetras |
| 2–4" (medium) | 3 gal | Guppies, mollies, dwarf gouramis |
| 4–6" (large) | 5 gal | Angelfish, rainbowfish, larger barbs |
| 6–10" (XL) | 8 gal | Oscars, discus, large cichlids |
| 10"+ (jumbo) | 15 gal | Arowanas, large plecos, pacu |
Standard Quarantine Protocol
| Phase | Duration | Treatment |
|---|---|---|
| Observation | Days 1–7 | Watch for symptoms. Maintain pristine water quality. |
| Prophylactic treatment | Days 8–21 | General anti-parasitic (e.g., Praziquantel) and/or antibacterial |
| Final observation | Days 22–30 | Confirm no symptoms. Match QT water parameters to display tank. |
Total recommended quarantine: 4–6 weeks for freshwater, 6–8 weeks for saltwater. Marine ich (Cryptocaryon) earns the longer window because its encysted stage can sit dormant for up to about 72 days before hatching.
Worked Example: Quarantining 6 Neon Tetras
Fish size: Under 2" (small). Gallons per fish: 1.5. Fish volume: 6 × 1.5 = 9 gallons. Base volume: 10 gallons. Recommended QT: max(10, 9 + 10) = 19 gallons → round to standard 20-gallon tank.
In practice, a 20-gallon long tank is ideal. The extra length gives small schooling fish room to swim, reduces stress, and provides stable water parameters.
Water Change Schedule During Quarantine
| Tank Size | Water Change Frequency | Volume per Change |
|---|---|---|
| 10 gallon or less | Every 2 days | 30% (3 gallons) |
| 11 to 20 gallon | Every 3 days | 25% (5 gallons on a 20) |
| Over 20 gallon | Twice per week | 20% |
QT Equipment You Actually Need
- Sponge filter (seeded from main tank for instant cycle)
- Adjustable heater
- Thermometer
- PVC pipe or terracotta pot for hiding
- Bare bottom, no substrate. Waste is visible, siphoning is instant, and medication has nothing to bind to.
- Lid/cover (stressed fish jump)
The mistake that undoes the whole thing
A quarantine tank has no established filter unless you gave it one. Move a fish into a bare 10-gallon with a brand-new sponge and you have put it in a tank that will spike ammonia within days, which stresses it far more than the disease you were watching for.
Keep a spare sponge filter running permanently in your display tank. It costs nothing to run and it is what makes a quarantine tank deployable on the day you need it rather than six weeks later.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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