Quarantine Tank (QT): What It Is and Why Every New Fish Needs One
A quarantine tank, usually shortened to QT, is a separate, bare bottom tank used to hold newly purchased fish, corals, or invertebrates for several weeks before they go into the main display. It gives you a window to watch for disease and, if something turns up, treat it in an isolated system instead of dosing medication into a tank full of established fish, plants, or corals.
What a Quarantine Tank Actually Looks Like
A functional QT does not need to be elaborate. Most hobbyists run a bare bottom tank with no substrate, so waste and any parasites are easy to see and siphon out, along with a simple sponge filter or hang on filter seeded with bacteria from an established tank, a heater, and enough hiding spots (PVC pipe, a plastic plant, an inexpensive decoration) that the fish is not sitting exposed under bright light for weeks. Size depends on what you are quarantining. A 40 to 75 liter tank covers most single fish or small groups; larger or more aggressive species need more room. Marine hobbyists often run a QT permanently rather than assembling one only when needed, since new marine stock arrives more often and marine parasites are harder to fully clear.
Why Quarantining Matters
Many of the most common aquarium diseases (ich, velvet, and internal parasites among them) do not show visible symptoms right away. A fish can look completely healthy in the store tank and still be carrying something that only breaks out days or weeks later, often triggered by the stress of shipping and a new environment. Treating disease once it reaches a display tank is far harder than treating it in a QT. Many medications harm shrimp, snails, and corals; copper based treatments in particular are lethal to invertebrates. Dosing a large established tank also costs far more in medication than dosing a small QT. Biological filtration can also take a hit from some treatments, which risks an ammonia spike on top of whatever disease you were trying to treat in the first place. A dedicated QT keeps all of that risk contained to one small tank instead of the whole system.
How Long to Quarantine and What to Watch For
A typical quarantine period runs two to four weeks for freshwater fish. Marine fish are usually held longer, four to six weeks is common, because marine parasites like ich and velvet have life cycles that need to be broken across more than one generation before a fish can be called clear. During that window, check daily for clamped fins, rapid breathing, white spots, flashing against decorations, or refusal to eat. Many hobbyists use the quarantine period proactively rather than reactively: a preventative treatment course for common parasites, run on a set schedule rather than waiting for symptoms to appear, catches infections before they ever reach the display. Feed normally throughout so you can judge appetite, and only move the fish to the main tank once the full quarantine period has passed with no symptoms.
Common Quarantine Mistakes
The most common mistake is skipping quarantine altogether because a fish “looks fine,” which defeats the point since the diseases QT exists to catch are usually the ones without early symptoms. A close second is running the QT without any cycled filtration, which turns a disease control tank into an ammonia problem within days. Adding new arrivals to an already occupied QT restarts the clock for everyone in it, since a fresh introduction can bring in something the earlier fish were never exposed to. And releasing a fish early because the tank “looks empty” and needs stocking undoes weeks of otherwise careful quarantine, since most outbreaks show up in the final third of the window, not the first.
FAQ
What is a QT tank?
QT is shorthand for quarantine tank, a separate tank used to hold and observe new fish before adding them to a main display or reef.
Do I really need a quarantine tank?
For anyone keeping fish long term, yes. It is the single most effective way to keep a disease outbreak, or an unwanted hitchhiker on new coral or rock, out of an established tank where treatment is far more disruptive and costly.
How long should a quarantine tank run?
Two to four weeks for freshwater fish, four to six weeks for marine fish, is the commonly used range. Extend it if any symptoms appear during the observation window.
Can I quarantine plants or corals too?
Yes. Coral and plant quarantine focuses more on hitchhikers such as pests, snails, or algae than on disease, and a shorter one to two week observation period is common practice.
Does a quarantine tank need a heater and filter?
Yes. It needs stable temperature and cycled biological filtration the same as any other tank; the only real difference is the bare bottom and minimal decor.
What size tank works for quarantine?
A 40 to 75 liter tank suits most single fish or small groups. Match the size to what you are actually quarantining rather than keeping one fixed size for every case.
Shop Quarantine & Treatment
See Marine Aquarium Medicine or Freshwater Medicines at IndieFur depending on your setup.