wikiHow to Quarantine Freshwater Fish

Quarantine tanks are essential to preventing disease outbreaks in fish tanks.[1] By having a quarantine tank on hand and knowing how to properly quarantine a new or sick fish, aquarium communities are protected from contagious diseases and the harsh chemicals sometimes used to treat them. Best of all, the tank can be stored when it's not needed!


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    Obtain the necessary supplies. A quarantine tank does not need to be expensive or look pretty. Used tanks work great if they are clean, or a basic 10 gallon aquarium will do. Particularly large fish require a tank big enough to comfortably house them for 14 days. Make sure the heater and filter are adequate for the size tank you are using.
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    Prepare the tank. Quarantine tanks do not use substrate (like gravel), so paint the outside bottom (not inside) of the tank black to give a sense of "bottom" for your fish or you could stick a black bag under it. Clean everything with hot water, never soap or other chemicals. If you are making a quarantine tank "just in case", then stop here, and store the tank until you need it. When ready to use it, double check that everything is clean and dust free, and continue to the next step.
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    Set up your quarantine tank. Fill the quarantine tank with water from your main aquarium. Add the hiding places, filter, heater, and aerator (either an airstone or air pump) and turn everything on. Test the chemical levels in the water and make sure nothing is out of balance. Try to keep the quarantine tank at the same temperature as your main tank, as you will eventually be transferring fish between the two. If your filter uses carbon, remove the carbon as it can lower the effectiveness of medications.
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    Cycle your filter. There are a few options for doing this:
    • Run your filter in your main aquarium alongside the existing filter for at least two weeks.
    • Put the media from the quarantine filter into the main filter for at least two weeks. Keep the media in your main filter until you need it, but replace it after using it in your quarantine tank. You don't want to contaminate your other fish, so cycle new media next time you need to quarantine a fish.
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    Transfer your fish. Changes in pH, temperature, and water hardness can be very stressful, so you need to be sure the fish is properly acclimated to the water in the quarantine tank. If quarantining a sick fish from an established aquarium, net the fish and place in the quarantine tank, provided you have used water with the same parameters and temperature as the water in your aquarium. If the parameters are different or you are quarantining new fish, slowly acclimate the fish to the water in the tank using one of the following methods:
    • Begin by floating the fish in the bag from the store for ten minutes, to equalize the water temperature. Next, add a small amount of tank water to the bag and wait another ten minutes. Repeat at least once. Finally, transfer the fish using a net into the tank. The bag-water never goes into the tank. This is a quick acclimation, and shouldn't be used on very weak or delicate fish.
    • Drip acclimation is a gradual process that is less stressful on the fish. Put the fish and its current water in a container big enough for at least twice the amount of water you have. Place this container lower than the quarantine tank, on a stable surface. Using tubing with a control valve (a valve with a screw in it to adjust the flow of water), place one end of the tube in the quarantine tank and suck on the other end to begin a flow of water, then place this end in the container with your fish. Adjust the flow to a few drips per second. When the water level in the container has doubled, remove half of the water. When the water level doubles again, your fish should be acclimated and can be moved into the quarantine tank. Dispose of any leftover water in the bucket.
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    Observe the fish. For the next two weeks, observe the fish closely for signs of illness. Feed only as much as they can eat in two minutes, twice daily. Test the water daily and perform water changes as needed.
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    Treat any illness. If a fish does become sick, diagnose the illness, possibly with help from an aquarium expert, and decide on a treatment. Extend the quarantine at least two weeks from the time you start treatment, to ensure the fish returns to health and no additional problems arise. Again, watch the fish and chemical levels closely during this time. Keep the fish in quarantine for at least a week after it appears healthy again.
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    Transfer fish to main tank. Once you are satisfied your fish is healthy, he may join the rest of your fish in the main tank. Use the slow transfer method again, as the chemistry in the quarantine tank is not going to be the same as your main tank after two weeks. Try not to mix any quarantine water into your main tank, as it could still contain medications.
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    Store the quarantine tank. If not adding any more new fish, and everyone else seems healthy, store it until needed again. Empty the water and wash everything with warm water (no soaps or chemicals!). Let it all dry, and store in a safe, clean place.


  • For very sick, weak, or young fish, use a sponge filter. These gentle filters can also be run alongside the filter in your main tank until you need them.
  • Always wash your hands before and after handling the tank and its components, and after handling medications and other chemicals.
  • Be sure to provide at least two hiding places. Fish with nowhere to hide, especially if they are coming from a pet store, can easily become stressed. Stress lowers the fish's ability to fight illness, and can lead to death.
  • Filters which hang on the back of the tank are probably the best for use in a quarantine tank. If your tank is large enough, you can even keep it running alongside your main filter in your primary tank. Then, when it is needed in the quarantine tank, the biological filter is all ready to go.
  • A quarantine tank can also be used for aquatic plants before adding them to the tank, to prevent outbreaks of snails and hydra. It is not, however, particularly useful for preventing bacteria or disease, as they are usually not observable on the plant. For this, a disinfecting rinse or soak may be preferable.
  • Tupperware containers and buckets may be used in place of a true aquarium, but make sure they are clean and chemical-free, and that they are both large enough for your fish and can accommodate a filter and heater.
  • Some aquarium enthusiasts recommend feeding new fish antibacterial medicated food while in quarantine. This helps eliminate any bacterial disorder they may be suffering from. If the fish are wild-caught, it may be a good idea to feed a combination anti parasitic food, which may take care of internal parasites as well as bacteria.[2]
  • Many stores have a one or two week guarantee on the health of their fish. If a new fish becomes ill or dies while in quarantine, try returning it for a refund/replacement. Most stores require you return the actual fish with a receipt, so keep it until the guarantee expires.


  • Immediately remove any dead fish from the tank.
  • Anything used as a hiding place should have all openings larger than the fish and smooth edges to prevent injury. Fish can easily become stuck attempting to squeeze through a too-small hole, and jagged edges can shred delicate fins. To test the edges, run a pair of nylons along all edges in multiple directions. If an edge snags the nylon, it can tear your fish's fins.
  • Some medications are deadly to certain fish, and some can be harmful to humans and other pets. Carefully read the labels before treating your fish or handling new chemicals. Many chemical testing kits contain caustic or toxic chemicals, so use caution when testing the water levels.

Things You'll Need

  • 10 gallon (37.9 L) fishtank with hood and light (larger if necessary)
  • Filter
  • Heater
  • Airstone or air pump
  • Chemical test kit that tests for pH, ammonia, nitrate and nitrite
  • Black paint/black bag
  • Simple hiding places such as clean PVC pipe
  • Fishnet, not the same one used in the main tank
  • Medications as needed to treat disease

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