How to Quarantine New Fish: Tank Setup, Water Tests and Timing

Seed a sponge filter in an established tank before you buy

The Merck manual suggests running the disinfected sponge filter in a healthy established tank “to inoculate the sponge with nitrifying bacteria”, and says: “This practice will help avoid the common water quality problems that occur in newly set up aquaria.” Without one, our dark start cycling guide covers another way to cycle a tank.

Set the tank up away from your main tank

Aquarium Co-Op advises: “To avoid cross contamination, place the quarantine tank in a different room away from your main display tanks if possible.” Leave the bottom bare, since Aqueon says “Do not use gravel or substrate”, and add the hides; Miller-Morgan, quoted in TFH, wants “nothing porous, like lava rock”. On a tub, Aquarium Co-Op prepares the lid by “drilling some holes for air flow” and cutting a slot for cables and airline tubing. TFH says to “keep the lighting low”.

Fill it with conditioned water at your species’ temperature

Aqueon: “Always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium.” Set the heater to the temperature your fish’s species needs (check its care sheet); Practical Fishkeeping, a fishkeeping magazine, notes that a separate tank lets you “match water conditions and temperatures easily”. Then move the seeded sponge filter across and start the air pump.

Test before the fish go in

Aqueon: “Test the aquarium water for pH, ammonia and nitrite before adding new fish.” UF/IFAS Extension, in an aquaculture publication, says: “In healthy ponds and tanks, ammonia levels should always be zero.” Aqueon, an aquarium maker, says: “The only safe nitrite level is zero.” In a tank with no fish yet, zero ammonia and nitrite show clean water, not a cycled filter. Aqueon says a quarantine tank “should be fully cycled” before it is used for new fish; seeding the sponge in step 1 is Merck’s way to help, and step 7’s tests show how it is doing. If either reads above zero, hold off on buying; see our guides to lowering ammonia and lowering nitrites.

Buy with care and go straight home

Aqueon advises letting fish that just reached the store “settle in for a week or more before purchasing”, then taking new purchases “straight home”. The CDC says: “Pick fish that are active in their tanks in the pet store.” and “Don’t select fish from a tank that has a high number of dead fish.”

Acclimate the fish into the quarantine tank

The CDC says to float the intact bag at the surface “for about 30 minutes to an hour”; Aqueon gives “at least 30 minutes” for pH and temperature. The sources differ on the release. The CDC says to open the bag and pour it gently, warns “Do not remove the fish from the bag with a small net as this may injure the fish”, and adds: “Try not to pour too much water from the bag into your aquarium at home because of the possible germs it may contain.” Aqueon is stricter: “Don’t allow store water to enter your aquarium when adding new fish.”

Test ammonia and nitrite through the whole period

For a newly set-up tank, API, a test-kit maker, says to test ammonia “several times a week for 4-6 weeks when you start a new aquarium”, and: “Water should be tested for nitrite every other day when your aquarium or pond is first set up”. For a new aquarium, Aqueon advises: “Stock your new aquarium gradually, feed sparingly and test water regularly until ammonia and nitrite levels stabilize at zero.” If ammonia rises, our ammonia guide covers the water side; if either reading rises, keep testing and ask a fish vet about the fish.

Watch the fish every day

The CDC: “Check your fish daily for signs of illness. A sick fish may swim abnormally, have red, brown, or white splotches, not eat, or be swollen.” If you see any of these, test the water at once, as UF/IFAS Extension advises, then “Talk to a veterinarian experienced in fish medicine”, as the CDC puts it. The American Association of Fish Veterinarians runs a Find a Fish Vet map; it says the map “only supports fish vets located in North America”.

Keep the quarantine gear separate and work on that tank last

Merck says: “Quarantined animals should be handled only when contact with all other animals is finished for the day.” Aqueon: “Have a separate net, siphon hose, algae scraper and other equipment for your quarantine aquarium, and never use them in your display aquarium.”

Wash up, and keep tank water out of the kitchen

Aqueon: “After working in your quarantine aquarium, scrub your hands and lower arms with an anti-bacterial soap before working in your display tank.” The CDC says: “Dump tank water and clean aquarium items in a bathtub, laundry sink, or outside instead of a kitchen sink when possible.” and “Clean and disinfect sinks and bathtubs after using them to dump aquarium water or clean aquarium items.” It adds: “People with weakened immune systems should let others in the household care for the fish tank when possible. If this isn’t possible, they should wear gloves.” “Don’t let children younger than 5 years old clean or handle aquarium items.”

Move the fish when the period ends

Merck says “Quarantine is not useful for some pathogens, but that does not mean it should not be done”; a quiet quarantine does not prove the fish are healthy, so if any of the CDC’s signs appeared, ask a fish veterinarian before moving them. Acclimate them into the main tank as in step 6.

Disinfect and dry the gear

Merck: “Once fish have cleared quarantine, the tank and equipment should be disinfected and stored dry until needed again.” Aqueon disinfects quarantine equipment “in bleach water” and says to “rinse well”. Rinse off organic matter first: Miller-Morgan cautions in TFH that “organics do cause bleach to break down”. The CDC’s bleach page is written for household surfaces, not aquarium gear; its safety points apply to the bleach itself. It warns: “Never mix household bleach (or any disinfectants) with any other cleaners or disinfectants. This can release vapors that may be very dangerous to breathe in.” It says: “Use regular unscented household bleach.” and “Most household bleach contains 5%–9% sodium hypochlorite. Do not use a bleach product if the percentage is not in this range or is not specified.” Neither Merck nor Aqueon gives a dilution for aquarium equipment, so this guide gives none. The CDC adds: “Follow the label directions on the bleach product. Check to see if you need to wear any protective equipment, such as gloves or eye protection.” Work with “good ventilation” indoors.