How to disinfect a fish tank after disease, a death or buying second-hand

To disinfect an empty fish tank, scrub it, soak or wipe every surface with a diluted household bleach solution for at least 20–30 minutes, rinse it several times, and finish with a dose of dechlorinator. Porous things such as gravel, filter sponges and resin ornaments can't be reliably disinfected, so replace them.

You don't always need to go that far. Many common parasites, such as ich, die out on their own once there are no fish. Full disinfection matters most for stubborn bacteria such as fish tuberculosis, for second-hand tanks with an unknown history, and before housing valuable or endangered fish.

Never use any disinfectant in a tank with fish, shrimp or snails in it.

When you need it

Full disinfection: after fish TB (mycobacteriosis), after an outbreak you couldn't identify that killed many fish, after septicemia or columnaris wiped out a tank, and on any second-hand tank, filter or decor.
Leaving it empty is usually enough: after ich or velvet. These parasites need a fish to complete their life cycle. With no fish, at normal tropical temperatures, ich generally dies out within about two to three weeks; run the tank fishless for a month to be safe. Most other fish germs fall to harmless levels after several weeks without hosts.
After a single death from old age or an accident: no disinfection needed. Remove the body, test water and do a water change (see dead fish).

What you need

Plain, unscented household bleach (sodium hypochlorite, usually 5–8%). Never "splash-less", scented or "with cleaners" bleach.
Rubber gloves, eye protection and old clothes. Work with windows open, away from any other tanks.
A bucket, clean sponges or cloths that have never touched soap, and a razor scraper.
Dechlorinator (water conditioner).
For fish TB: 50–70% ethyl alcohol (ethanol). The fish TB studies tested ethyl alcohol; many rubbing alcohols are isopropyl instead, so check the label.
White vinegar for hard-water stains.

Step by step

Empty the tank. Move any surviving fish to another tank. Bag the old gravel, filter media and porous decor for disposal.
Scrub everything. Remove algae, slime and dirt with water and a scraper. Disinfectants work badly through dirt and biofilm.
Remove hard-water stains if you like: soak a cloth in white vinegar, lay it on the white crust for 15–30 minutes, then scrape. Use plastic scrapers on acrylic tanks.
Mix the bleach solution. A widely used ratio is about 6 fl oz of bleach per US gallon of water, which is roughly 50 mL per litre, or 1 part bleach to about 20 parts water. Mix in a bucket, never in a closed space, and never with vinegar, ammonia or other cleaners, which release toxic gas.
Apply and wait. Fill the tank with the solution, or wipe every inside surface so it stays wet. Soak nets, hoses, heaters (unplugged), filter housings, siphons and plastic plants in a bucket of the same mix. Leave it for at least 20–30 minutes. Lab tests found bleach needed about 20 minutes to kill the fish TB bacterium.
Rinse thoroughly. Empty the solution and rinse with fresh water three or four times.
Neutralise. Fill the tank with tap water and add dechlorinator at two or three times the normal dose to neutralise any leftover chlorine. Soak small equipment in a bucket of the same mix. Leave for an hour.
Dry and sniff. Drain and let everything air-dry for at least a day. If you can still smell bleach, rinse again.
For fish TB only: after bleaching, wipe or spray all surfaces with 50–70% ethyl alcohol and let it evaporate. In lab tests, ethyl alcohol killed the fish TB bacterium within about a minute, while bleach was only moderately effective, even at high strength. Experts disagree on how far to go: one fish-health reference argues that fully sterilising a tank against TB is impractical, while aquaculture vets recommend bleach followed by an alcohol wipe. Doing both gives you the best odds.
Set up and cycle again with new substrate and media before adding fish. See cycling.

Hydrogen peroxide

Ordinary 3% hydrogen peroxide is a gentler option for wiping surfaces and soaking equipment, and it breaks down into water and oxygen. It's useful when you don't want bleach near rubber seals or wood, but there is less evidence for it against tough germs like mycobacteria, so don't rely on it after fish TB. Rinse afterwards.

What you can't disinfect well

Gravel and sand: germs hide deep in the bed. Replace it after TB or a severe outbreak. Large, smooth gravel can be bleached and rinsed if you must keep it.
Filter sponges and ceramic media: replace them. They're full of biofilm.
Driftwood and porous rock: wood soaks up bleach and releases it later. Some keepers prefer hydrogen peroxide or just replace wood after serious disease.
Resin ornaments with cracks, live plants and anything with cavities: replace them, or quarantine plants in a separate tank.

Common mistakes

Using scented or "cleaning" bleach, or soap. See soap in the tank.
Not rinsing well enough. Even a faint bleach smell can kill fish. See chlorine.
Bleaching a running tank, or spraying near another tank: fumes and drips can harm fish.
Keeping the old filter sponge "for the bacteria" after a serious outbreak.
Assuming bleach alone has dealt with fish TB.

Products that help

Unscented household bleach: the standard all-round disinfectant for empty tanks and gear.
50–70% ethyl alcohol: the follow-up for fish TB.
Dechlorinator: neutralises leftover chlorine after rinsing.
Separate nets and siphons for each tank: stop germs moving between tanks.
Aquatic disinfectant dip for nets (between tanks): handy, but check the label; some popular aquatic disinfectants don't kill mycobacteria.
Plastic razor scraper: removes crust without scratching.

How to prevent the need

Quarantine new fish for 4–6 weeks. See quarantine.
Use separate nets for each tank, or dip and dry them between tanks.
Remove sick or dead fish quickly.
Wash hands and arms before and after working in each tank.

Protect yourself

The bacteria behind fish TB can infect people through cuts and scrapes, causing a skin infection known as fish tank granuloma. Wear gloves when cleaning a tank that held sick fish, cover any cuts, and see a doctor if you get a sore, lump or rash on your hands that won't heal, telling them you keep fish.

When to get help

Talk to a fish vet if you're not sure what killed your fish, before rebuilding a tank for valuable or endangered stock, or if deaths continue after you've cleaned up. A necropsy (an examination after death) can tell you whether full disinfection is needed.

In the game

Multiple Tank Syndrome doesn't have a disinfection card yet. The Quarantine tank card is the closest: it's the habit that saves you from ever needing a full strip-down.