Quarantine tank setup: what you need and how long to wait

A quarantine tank is a small, bare, separate tank with a heater, a cycled sponge filter, a hide and a lid, where new fish live before joining your main tank. Keep them there for at least 30 days, and 4–6 weeks counted from the last sign of illness is the more cautious rule. Watch eating, breathing and droppings daily, change water on schedule, and treat only what you can identify, following the product label.

Quarantine means keeping new fish in their own tank, away from your established fish, long enough for hidden illness to show up. A basic setup costs little, and it can save a main tank full of fish from ich, flukes or bacterial infections that new arrivals often carry.

Why quarantine new fish

Shipping and store handling stress fish, and stress is what turns a quiet carrier into a sick fish. The University of Florida’s extension publication on ich says fish that look healthy can break out in active white spot after shipping, and in warm water the signs usually appear one to three weeks after arrival. A fish put straight into your display tank takes that outbreak with it.

A small tank also makes treatment practical. You need less medication in 10 gallons than in 55, and you don’t expose live plants, snails or your filter bacteria to it.

Quarantine has limits. The Merck Veterinary Manual notes it is best at catching external parasites and some internal ones that show in droppings. It cannot reliably find mycobacteria, many viruses or microsporidia, so choose healthy-looking fish from a clean source as well.

What size quarantine tank do you need?

Pick the smallest tank that still gives the fish room to swim and keeps water stable.

Fish you are quarantining	Tank size
One betta, or a few shrimp or nano fish up to 1.5 in (4 cm)	2.5–5 gallons (9–19 L)
A small group of tetras, rasboras, danios, corydoras or guppies	10 gallons (38 L)
Larger fish such as angelfish, gouramis or small cichlids	20 gallons (76 L)
Big fish such as large cichlids, goldfish or plecos	A 20–40 gallon (76–151 L) tank or a large tub sized to the adult

A clear plastic storage tub with a lid is a cheap, common alternative to glass, and its clear sides let you watch the fish from all angles. Before buying, check what you plan to add with the 10-gallon tank page or the stocking calculator. A betta can quarantine in as little as 2.5 gallons.

What you need

A heater and thermometer. Match your main tank, usually 76–80 °F (24–27 °C) for tropical fish. Use the heater size calculator if you aren’t sure of the wattage.
A sponge filter and air pump. Sponge filters are gentle, cheap, and easy to move or clean. Strong flow tires small fish.
No substrate. A bare bottom lets you spot droppings and leftover food, and there is nothing for waste and medication to hide in.
Hides. A short piece of PVC pipe or a small terracotta pot gives fish somewhere to settle. Frightened fish that cannot hide stay stressed.
A lid or cover. Stressed fish jump, and a lid also keeps the tank quieter.
Dedicated tools. A net, siphon and bucket used only on this tank. Wash your hands after working on it, and do quarantine chores last.
Water conditioner and test kits for ammonia, nitrite and pH.

How to keep it cycled

New fish make ammonia, and a brand-new tank has no bacteria to remove it. You have two easy fixes.

Seed the filter. Run a spare sponge filter (or a bag of filter media) in your main tank for a few weeks. Move it into the quarantine tank when fish arrive and the bacteria come with it. Aquarium Co-op and the Merck Veterinary Manual both describe this, and Merck says to take the seed from a disease-free tank.
Cycle it. Run the tank with a source of ammonia as in the guide to cycling a fish tank. This takes weeks, so it only works if you plan ahead.

Don’t leave the quarantine tank running empty for months. Without ammonia the bacteria starve, so store the sponge in the main tank instead and set the tank up when you need it.

Set it up in six steps

Pick a spot. Ideally in a different room from your main tank, or at least far enough away that nothing splashes between them.
Fill with dechlorinated water at the temperature of your main tank. See tap water if you’re unsure how to treat it.
Install the sponge filter, heater and thermometer. Add the hide, and run everything for a few hours to confirm the temperature holds.
Test ammonia and nitrite. Both should read 0 before fish go in.
Add the fish using the method in the guide to acclimating fish. Do not pour the bag water into the tank.
Turn the light off for the first evening and wait until the next day to feed.

How long to quarantine

The most widely cited minimum is 30 days. It appears in the UF/IFAS ich publication and in the Merck Veterinary Manual, which says longer may be needed. Aquarium Co-op suggests 4–6 weeks, counted from the last sign of illness or death in the tank, which is the safer rule for fish from large stores or online sellers.

Situation	Suggested hold
Fish from a trusted local breeder, all healthy	30 days
Fish from a chain store or an unknown source	4 weeks, up to 6
A fish gets sick or dies during quarantine	Restart the count once everyone looks well
You treated a disease	Count from the end of treatment, as the label directs

If you can, add all the fish for one batch on the same day. Adding a new fish partway through restarts the clock for the whole tank.

What to check each day

Spend a few minutes at feeding time watching each fish, with the light on.

Eating. Does each fish take food, and does anything spit it out?
Breathing. Gills should move calmly. Gasping at the surface or fast gill movement means you should check ammonia and oxygen.
Behavior. Hiding is normal for a few days. Lying on the bottom, flashing against decor, or clamped fins that persist are not.
Body. Look for white spots, fuzz, red streaks, torn or frayed fins, cloudy eyes, or a swollen belly.
Droppings. Long, stringy, white droppings are worth noting.

Test ammonia and nitrite every day or two in the first week, then weekly. Change 25–50% of the water each week with temperature-matched, dechlorinated water. See water changes for how, and take action as in the ammonia guide if you ever see a reading above 0.

Should you medicate new fish preventively?

Experienced keepers split on this, so here are the common views.

Watch and wait. Use the time to observe, and treat only when something shows up. This avoids unneeded drugs, and Merck strongly discourages medicating without a diagnosis because it can encourage resistant bacteria.
Treat by source. Aquarium Co-op suggests watching fish from trusted sellers, and treating fish from large chains or online sellers with a standard preventive routine. Their routine uses over-the-counter antibiotic, parasite and ich products.
Treat on a vet’s advice. For valuable fish or repeat outbreaks, an aquatic vet can examine skin and gill samples and tell you what is actually there.

If you do medicate, read and follow the product label for dose, length, and how often to change water. Take activated carbon out of the filter during treatment, since it absorbs medication. Never combine products unless both labels allow it, and remember that some medications are unsafe for scaleless fish, shrimp, snails and plants. This guide doesn’t give doses for prescription drugs.

Using it as a hospital tank

The same setup is a hospital tank. When a fish in your main tank looks sick, move it into the bare tank so you can treat it, watch it, and keep the rest of the tank unmedicated.

Match the temperature before you move it, and keep the water clean, because fish under stress are more vulnerable.
For white spot, treat the whole display tank as well, because the parasite is already there: see how to treat ich.
For torn or rotting fins, clean water does most of the work: see fin rot.
If several fish are dying or the cause is unclear, start with why fish die and call an aquatic vet.

Moving fish to the main tank

When the fish have eaten well, behaved normally and shown no signs of disease for the full period, move them across.

Match temperature, then net the fish across. Skip the long drip: both tanks use the same tap water.
Don’t tip the quarantine water into the main tank.
Turn the main tank lights down for the first evening.
Add them in small batches, and test ammonia and nitrite the next day.

Cleaning the tank afterward

The UF/IFAS publication warns that ich cysts spread on shared nets, hoses and decor. When the last fish leaves, empty the tank and wash the tank and tools with hot water and no soap. Disinfect equipment as the Merck Veterinary Manual suggests, rinse well, and store it dry. Keep the sponge filter wet and running in the main tank if you want it ready for the next batch. If you will buy fish again soon, you can instead leave the quarantine tank running.

Common mistakes

Skipping the cycle. An uncycled quarantine tank poisons fish faster than disease does.
Cutting the time short. Many outbreaks appear in weeks two and three.
Using the same net or siphon on both tanks. It carries parasites straight across.
Adding fish partway through. The count restarts for everyone.
Pouring bag water or quarantine water into the main tank.
Medicating “just in case” without reading the label, or leaving carbon in the filter.
Overcrowding the small tank. More fish means more ammonia and more stress.

Frequently asked questions

What size should a quarantine tank be?

Match it to the fish: 2.5–5 gallons (9–19 L) for a single betta or a few nano fish, 10 gallons (38 L) for most community fish, and 20 gallons (76 L) or more for large or schooling species. Small tanks use less medication, but they swing faster in ammonia, so never go smaller than the fish can turn around in comfortably.

Do I need a quarantine tank for a betta?

If the betta is going into a tank with other fish or shrimp, yes. If it will live alone in its own tank, that tank can act as its quarantine: watch it closely for a few weeks and keep its nets and tools separate from your other tanks.

Can I keep a quarantine tank running empty?

You can keep the filter running in your main tank instead and move it over when needed. An empty tank with a filter running has no ammonia to feed the bacteria, so they slowly die off. Seeded sponge filters kept in a healthy tank are the easiest fix.

Should I medicate all new fish in quarantine?

Hobbyists disagree. Some keepers treat fish from unknown sources with a set routine, while veterinary guidance discourages medicating without a diagnosis because it can breed resistant bacteria. Whichever you choose, follow the label and never mix medications unless the labels allow it.

Can I use the quarantine tank for sick fish?

Yes, and it is the main reason to keep one set up. Moving a sick fish to a bare, small tank lets you treat it with less medication and without exposing plants, snails or the bacteria in your main filter.