How to quarantine new fish

Updated 2026-06-145 min read

New fish are the most common way disease and parasites enter an aquarium. A fish can look perfectly healthy in the shop and still carry ich, velvet, or internal parasites that erupt once it is stressed by the move.

Quarantining new fish in a separate tank for a few weeks lets problems surface where they are easy to treat — before they reach your established community. This guide shows how to do it.

Steps

Set up a separate quarantine tank
Use a simple bare-bottom tank with a heater, a gentle filter, and a hiding spot. It does not need to be fancy — just stable, cycled water and a way to medicate.
Keep a sponge filter running in your main tank year-round so you always have cycled media ready to seed a quarantine tank fast.
Acclimate the new fish gently
Float the bag to match temperature, then slowly mix in tank water over 20–30 minutes before releasing the fish. A gentle transition reduces shock and stress.
Observe for two to four weeks
Keep new arrivals in quarantine for at least two weeks, ideally four. Watch daily for spots, frayed fins, clamped fins, odd swimming, or loss of appetite.
Treat problems in isolation
If disease appears, treat it in the quarantine tank where dosing is easier and your main tank stays untouched. This is the whole payoff of quarantining.
Keep the water pristine
A small bare tank fouls quickly. Test often and do frequent water changes so the fish recovers from travel stress in clean, stable conditions.
Move healthy fish to the main tank
Once the quarantine period passes with no signs of illness, acclimate the fish to your display tank and add it. Repeat the routine for every new arrival.