Quarantine means keeping new fish in a separate tank before they join your main aquarium. It is the simplest way to keep diseases such as ich and columnaris out of an established tank.
How long to quarantine
The Merck Veterinary Manual gives 30 days as the minimum quarantine period, and says longer may be needed (Management of Aquarium Fish). Most problems that new fish bring with them show up within that time.
What you need
- A 10-gallon or larger tank with a lid, heater and thermometer.
- A sponge filter, ideally run in a healthy, established tank for a few weeks first so it carries beneficial bacteria.
- An air stone for extra oxygen.
- A water test kit for ammonia and nitrite.
- A separate net, bucket and siphon that never touch the main tank.
This is the same equipment as a hospital tank, so one setup can do both jobs.
Step by step
- Set up the tank a few days before you buy, so the temperature is stable.
- Acclimate the new fish slowly to the quarantine water, as you would for the main tank.
- Watch daily for the signs below, and note how well each fish eats.
- Test ammonia and nitrite every couple of days, and do partial water changes as needed.
- Feed lightly and remove uneaten food.
- Keep everything separate. Work on the main tank first and the quarantine tank last, and wash your hands between them.
What to watch for
| What you see | What it may be | Read |
|---|---|---|
| White spots like grains of salt | Ich (a parasite) | Ich vs bacterial infection |
| Ragged, receding fins | Fin rot | Fin rot |
| Grey-white patches on the back or mouth | Columnaris | Columnaris |
| Swollen belly, raised scales | Dropsy | Dropsy |
| Not eating, hiding, gasping | Stress or poor water | Test the water first |
Don’t add antibiotics “just in case”
Treating healthy fish with antibiotics doesn’t prevent disease. It stresses the fish, harms the filter’s beneficial bacteria and helps resistant bacteria develop. Use medicine only when you see signs and know what you are treating; How to choose an aquarium antibiotic explains how.
Ending quarantine
Move the fish to the main tank only after they have eaten well and shown no signs for the full period. Quarantine won’t catch everything: Merck notes that some infections, such as mycobacteria and many viruses, usually aren’t found this way. Afterwards, clean and disinfect the quarantine tank and store it dry.
Browse all fish antibiotics, compared by ingredient, strength and count.
Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.