The short answer
Quarantining new fish for 2–4 weeks in a separate tank is the single best way to protect your main aquarium. A new fish can look perfectly healthy in the shop yet be carrying a disease or parasite that only shows up once it’s stressed by moving. Quarantine catches that before it reaches your established, healthy stock — where an outbreak is far harder to control.
Why a separate tank matters
Adding fish straight into your display tank is the most common way hobbyists wipe out an otherwise thriving community. Once a parasite or infection gets into an established tank it can spread through every fish and hide in the substrate and filter. A modest quarantine tank — even a simple heated, filtered bin or spare aquarium — keeps any problem contained where you can watch and treat it without risking everyone else.
How to do it simply
- Give it 2–4 weeks. Most common issues surface within that window.
- Keep it low-stress: stable temperature, gentle filtration, hiding spots and dim light.
- Watch closely for spots, clamped fins, unusual breathing or not eating.
- Only move fish across once they’ve eaten well and looked healthy for the full period.
If you can’t quarantine
Not everyone has a spare tank, and that’s okay — just lower your risk other ways. Buy from a reputable source with clean, well-kept display tanks, avoid buying from a system where any fish look sick, acclimate slowly (see how to acclimate new fish), and observe new arrivals carefully for their first few weeks.
Whichever route you take, keeping your main tank stable makes fish more resilient — stay on top of water testing and regular aquarium maintenance. If a new fish does show worrying signs, see how do I know if my fish is sick? and, for anything serious, ask a vet or experienced fishkeeper rather than self-diagnosing.