Quarantining new fish: a 4-week protocol

A new fish from the shop is a potential carrier of ich, gyrodactylus, flexibacter, and a dozen other pathogens. Shop tanks see hundreds of fish through one volume of water; disease outbreaks are routine.

Quarantine — a separate tank where fish stay for 3–4 weeks before going into the main one. It's the best insurance against an epidemic in your expensive, established aquarium.

Why quarantine

• Most parasites become visible within 1–3 weeks.

• The fish adapts to your water parameters without the stress of tankmates.

• You can prophylactically treat for common infections.

• If the fish does get sick — easier to treat in a small tank.

Quarantine tank gear

• Volume: 20–40 L. A plastic tub works.

• A simple sponge filter — air-lift is best.

• A heater.

• Darkness or dim light.

• Minimal decor — a few PVC-pipe hiding spots, no substrate (easier to keep clean).

• Water tests.

Pre-cycling the biofilter

The biggest problem with a quarantine tank is the lack of a mature biofilter. Solutions:

• In advance (3–4 weeks ahead), run the sponge filter inside your established tank.

• When adding fish — move the sponge to the quarantine. Instant biofilter.

• Alternative — daily 30 % water changes.

The 4-week protocol

Week 1: adaptation

Minimum stress. No medication. Observe. Water parameters close to the shop's (if you know them).

Week 2: prophylactic treatment

With no symptoms — a prophylactic bath:

• NaCl salt 1–3 g/L for 5 days — against ectoparasites (NOT for corydoras, tetras).

• Alternative — a light course of antiparasitic (Esha Exit, JBL).

Week 3: test

Water change, clean tank. Watch for symptoms — flashing, coating, hanging at the surface?

Week 4: finale

With no symptoms — gradual transfer into the main tank. Acclimate to main parameters: 1–2 hours of drip-acclimation, then release.

When to extend quarantine

• Symptoms appear — treat and reset to "week 0."

• Fish is weak, eats poorly — add a week to recover.

• Any doubt — better an extra week than an epidemic in the main tank.

What not to do

• Don't add fish to the main tank "to try it out" — they will infect the whole stock.

• Don't prophylactically dose antibiotics — useless and harmful to the microbiome.

• Don't run a one-day "quarantine" — that's not quarantine.

Quarantine is insurance the price of one tank, protecting a main tank worth thousands. Anyone who has ever lost a whole stock to imported ich runs quarantine for the rest of their days.