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Biosecurity

Quarantine: reducing disease-transmission risk

A separate quarantine tank reduces the risk of introducing disease and lets you observe new fish closely. It cannot guarantee freedom from infection.

2 min readUpdated September 25, 2026By FishicianPublished August 24, 2026How we source

The short answer

  • Healthy-looking isn't healthy. Many parasites are microscopic or spend part of their life off the fish.
  • Stress wakes up infections. Transport can turn a hidden problem into a visible one days later.
  • Quarantine is also for watching. In a bare tank you can see it eat and breathe normally.
  • Don't medicate by default. Avoid blanket medication. A veterinarian may recommend targeted preventive treatment based on the species and risks.
  • Length is a judgment, based on species, temperature and disease risks. Some infections remain hidden; new arrivals restart the observation period.
A molly miller blenny resting on a white PVC shelter in a quarantine tank
A blenny in a separate quarantine tank with a simple PVC shelter.Photo: Scartella cristata Quarantine 2 — Coughdrop12 · CC BY-SA 4.0. Resized for display.
Quarantine from shop to display
  1. Check the sourceWhole system, not just one fish
  2. Set up QTSeeded sponge, own equipment
  3. AcclimateNo shop water in
  4. Observe dailyEating, breathing, skin, fins
  5. Move to displayComplete the planned quarantine; assess health

Before you buy

  • Dead or distressed fish in connected tanks, poor water or scratching fish are warnings about the whole system.
  • Check breathing, balance, body condition, skin, fins, eyes and appetite. Ask to see it eat.
  • Confirm the species and its adult needs first. See stocking.

Set up the quarantine tank

  • Separate everything: nets, siphons, buckets and towels just for QT.
  • A mature sponge filter from a healthy tank can provide biological filtration, but its capacity must be checked with ammonia and nitrite tests.
  • Simple: easy-to-clean housing with shelter, stable temperature, gentle flow and a lid. Provide suitable substrate for species that need it.
  • Test more often than your display. Small, lightly filtered tanks spike quickly.

What to watch each day

  • Breathing rate, gill colour, swimming and balance.
  • Appetite: does it actually swallow food?
  • Colour, skin, fins, eyes, droppings, and any scratching or flashing.
  • Record with dates so you can tell a trend from a one-off. See warning signs.

Why not treat just in case

  • It exposes a healthy fish to risk, can harm the biofilter and can breed resistance.
  • Copper and hydrogen peroxide have narrow margins and kill many invertebrates.
  • Can't quarantine? Buy from a source you can evaluate, add one fish at a time, and test and watch more after each addition.

Moving into the display

  • Complete the planned observation period and any directed treatment before transfer; normal appetite alone does not clear quarantine.
  • Add it at a quiet time, consider rearranging territory, and have a separation plan ready.
  • Keep watching closely: the first days in a stocked tank are their own stress event.

Sources

Each source is listed with the part of this guide it supports.