The short answer
Test the tank first: Aqueon says to check pH, ammonia and nitrite before adding new fish. Float the sealed bag, mix in small amounts of tank water (sources differ on how long), and keep store water out (the CDC says pour gently, not much). Add a few fish at a time (Aqueon).
These steps come from freshwater guides.
This guide gives no medication or treatment advice. If a new fish looks sick, test the water and talk to “a veterinarian experienced in fish medicine”, as the CDC, a federal agency, puts it.
Before You Bring New Fish Home
- Test the water. Aqueon, an aquarium maker: “Test the aquarium water for pH, ammonia and nitrite before adding new fish.” For readings above zero, see our guides to lowering ammonia and lowering nitrite.
- In a new tank, plan the cycle. Sources describe two ways, fishless or with a few fish; our cycling guide covers both, and our setup guide the steps before. Aqueon, which starts with a few fish, warns: “Resist the temptation to add too many fish at once! Your filter won’t be able to process a lot of waste at first and this could cause harmful ammonia and nitrite to rise to unsafe levels.” The Merck Veterinary Manual, a veterinary reference, says adding fish slowly over months “can result in high ammonia or nitrite concentrations if not done carefully, and this practice can be inhumane.”
- Pick healthy fish. The CDC: “Pick fish that are active in their tanks in the pet store.” It warns against a tank with many dead fish. Aqueon: “Never purchase fish that have just arrived at your local aquarium store.” It says to let them “settle in for a week or more before purchasing”, and to research each species first; our beginner fish guide compares ten.
- Quarantine if you can. Merck and Aqueon give 30 days as the minimum (Merck: “longer periods may be necessary”); The Spruce Pets, a pet-care publication, and LiveAquaria, a livestock retailer, suggest two weeks.
What You Need
- A test kit for pH, ammonia and nitrite.
- A small net, a bucket or bowl, and a measuring cup.
- Water conditioner for the top-up water.
- For the drip method: airline tubing and, as Aqueon says, “a flow regulator”.
The picks below come from our aquarium test kit roundup and water conditioner comparison.
| # | Preview | Product | Price | Link |
|---|---|---|---|---|
| 1 |  | API Freshwater Master Test Kit 800-Test Freshwater Aquarium…Test kit | Check on Amazon | |
| 2 |  | Aqueon Fish Tank Water Conditioner Bottle, 16 Oz | Water…Water conditioner | $11.49 Prime | Check on Amazon |
How to Acclimate and Add New Fish, Step by Step
Spruce acclimates fish into a quarantine tank the same way; our quarantine guide covers that tank.
Go straight home
Aqueon: “Go straight home with new fish. In extremely cold or hot weather, transport fish in an insulated cooler or Styrofoam shipping container. Do not place fish in the trunk of your car.”
Turn off the light and feed the resident fish
Aquacadabra, a UK aquarium retailer, says that “having a warm light next to the bag may cause the water in the bag to heat up, which could potentially harm your fish.” LiveAquaria says to feed the tank first, which it says will help reduce aggression toward the newcomers.
Float the sealed bag
To match temperature, the CDC floats it “for about 30 minutes to an hour”, Aquacadabra 20 minutes, Spruce 15 (table below). For bettas, Aqueon recommends “the floating acclimation method”.
Open the bag and add tank water a little at a time
Aqueon rolls the top outward into “a floatation ring”, then adds “small amounts of tank water to the bag every few minutes for half an hour”. Spruce first tests the pH of the bag water and the tank water and notes the difference, then adds 1/2 cup every 15 minutes for 1 hour at a difference of 0.1 to 0.3, or 2 hours at 0.4 or more, “or until the pH is equal”. If the bag fills, Aquacadabra pours some out into a separate container. LiveAquaria: “Never place an airstone into the shipping bag when acclimating your new arrival. This will increase the pH of the shipping water too quickly and expose your new arrival to lethal ammonia.”
Release the fish with as little store water as you can
Aqueon: “Never allow store water to enter your aquarium, discard it after you release the fish.” Sources differ on how. Aqueon nets the fish, and Spruce and FishLore, a fishkeeping community site, say to use a small net, which the CDC warns against: “Do not remove the fish from the bag with a small net as this may injure the fish.” The CDC says to “carefully open and gradually pour the bag”, and: “Try not to pour too much water from the bag into your aquarium at home because of the possible germs it may contain.” If you have only a large net, Spruce pours the bag through it over a bucket; FishLore does the same when fish are hard to net.
Discard the bag water and top up
In freshwater tanks, Spruce tops up with “dechlorinated tap water”. The CDC says to dump aquarium water and clean aquarium items in a bathtub, laundry sink or outdoors rather than a kitchen sink when possible, and to clean and disinfect the sink or tub afterward. Wash your hands with soap and running water, as the CDC advises. It also says: “Wear gloves when cleaning or caring for your aquarium, especially if you have cuts or wounds on your hands or if you plan to work with rough rocks or spiny fish.” “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.”
Leave the light off while they settle
Aqueon says “a few hours”; LiveAquaria “at least four hours” for its arrivals. Aqueon: “Do not tap on the glass or suddenly turn on the aquarium light in a dark room.” (See why tapping on the glass is discouraged.)
Watch the new fish and test the water
For the first few days, Aqueon advises checking that they are eating, not harassed and show no signs of disease. In a new tank, it says ammonia and nitrite “may begin to rise until the nitrifying bacteria in your filter catch up”; it advises water changes or chemical filtration media to keep them from dangerous levels, and adds: “If in doubt, consult your local aquatic expert for assistance.” The CDC: “A sick fish may swim abnormally, have red, brown, or white splotches, not eat, or be swollen.” If so, test the water, separate it, talk to a fish vet.
Drip Acclimation for Large or Sensitive Fish
Aqueon uses the drip method for fish too large to acclimate while floating and for sensitive fish.
Check the time in the bag, then empty the bag into a clean bucket
Aquacadabra drips only fish bought from a local shop that are home “in less than 3-4 hours”: it says fish raise the bag’s carbon dioxide and ammonia, and opening it after longer raises the pH. Aquacadabra: “The sudden change can cause stress to your fish as well as other illnesses and possibly death to your marine life.” It gives no other method for longer trips.
Siphon tank water into the bucket through airline tubing, slowed with a knot or valve
FishLore aims for “drips every one to two seconds”; Aquacadabra for “about 2-3 drops per second”. Aquacadabra says to “wait about 30 minutes to 1 hour” until the bucket gets full, then pour out about 50% and run a second round.
Drip, then move the fish
FishLore drips “for an hour or so” for most freshwater species, against Aquacadabra’s two rounds. Discard the bucket water as in step 6.
How Long to Acclimate New Fish: What Each Source Says
| Source | Time | How the fish go in |
|---|---|---|
| CDC (federal agency) | Float “about 30 minutes to an hour” | Pour gently; no small net |
| Aqueon (maker) | “at least 30 minutes” for pH and temperature | Net; no store water |
| The Spruce Pets (publication) | 15-minute float, then 1 or 2 hours by pH difference | Small net, or pour through a large net |
| FishLore (community site) | About an hour, by cup or drip | Small net, or pour through a net |
| LiveAquaria (retailer) | “no longer than one hour” in total, for its shipped arrivals | Not stated |
The sources give no single time. Aqueon says to spend “extra time with sensitive species or if the store’s water chemistry is significantly different from yours”.
Adding New Fish to a Tank With Other Fish
- Change the layout. LiveAquaria rearranges decorations before new fish go in, to break up established territories. Aqueon says established fish occasionally act aggressively toward newcomers, “especially true among cichlids”, and, when you start stocking, suggests one or two new decorations per new fish.
- Add a few at a time, spaced out. LiveAquaria suggests trying to add more than one at a time to an established tank, which it says minimizes the chance of one fish being singled out and harassed. In a new tank, Aqueon says to “only buy a few fish at a time. Wait at least a week between new fish additions.” Aquacadabra suggests “around 2 weeks” and says never to double the number of fish in the tank.
- Mind temperament. LiveAquaria advises adding smaller, more passive fish first and larger, more territorial ones later. The CDC notes that male bettas “will fight” if kept together.
Moving Fish to a New Tank
A new tank needs the cycling choice above; Aqueon says seeding it with gravel or filter media “from an established, healthy aquarium will help speed up the cycling process.” For moving fish between your own tanks, the sources here give no steps.
Mistakes and Fixes
| Mistake | Fix | Source |
|---|---|---|
| Only floating the bag | Mix in tank water too | FishLore |
| A fish looks sick | Test the water, separate it, talk to a fish vet | CDC (separate, vet) |
| A fish dies | Remove it promptly | CDC |
Our guide to why fish die in an aquarium covers the water causes.
Ready to buy?
Our Water Care & Testing section gathers test kits and conditioners; our Tank Setup & Care section has more guides.
Frequently Asked Questions
Q. How long can fish stay in the bag?
No source here gives a safe maximum. FishLore goes straight home “to avoid ammonia accumulating in the bag (in the form of fish waste)”; Aquacadabra drips only fish home in under 3-4 hours; LiveAquaria caps its shipped arrivals’ acclimation at one hour.
Q. Can I put new fish straight into the tank?
Aqueon: “don’t just dump them into the tank!”
Q. How long does a new fish take to settle in?
No source here gives a set time; Aqueon: “Your new fish may hide at first.”
Q. Can fish recover from new tank syndrome?
Whether a fish recovers is a question for a fish vet. The Merck manual calls new tank syndrome “a water quality problem that usually occurs within the first 6 weeks after a new fish hobbyist sets up a tank.” Test ammonia and nitrite.
Sources (13)
- CDC, Healthy Pets: Fish (federal agency)
- Merck Veterinary Manual, Management of Aquarium Fish (veterinary reference)
- Aqueon, Aquarium Do’s and Don’ts (maker)
- Aqueon, Freshwater Aquarium Disease Prevention (maker)
- Aqueon, How to Help Keep Your Pet Fish Alive (maker)
- Aqueon, What to Expect in the First 60 Days (maker)
- Aqueon, Freshwater Aquarium Water Quality (maker)
- Aqueon, Ultimate Fishkeeping Guide (maker)
- The Spruce Pets, Adding New Fish to an Aquarium (pet-care publication)
- LiveAquaria, New Arrival Acclimation Guide (retailer guide)
- LiveAquaria, Introducing New Fish to Your Aquarium (retailer guide)
- FishLore, Acclimating Fish to Your Aquarium (fishkeeping community site)
- Aquacadabra, How to introduce new fish into an aquarium (UK retailer blog)
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