How To Acclimate New Fish To An Aquarium


Getting new fish is an exciting event. So much work goes into setting up the perfect home for your new finned friends that when the big day comes to buy your fish, it can be easy to rush the process. Acclimating new fish to an aquarium takes time and care. Rushing the process of acclimating fish can be disastrous. Give your fish the best chance at a long, healthy, and happy life by following this article’s advice.

Before You Buy

Even if you strictly follow this article’s advice, you’re doomed from the start if you purchase inappropriate or sick fish. Choosing fish that are healthy, will get along with each other, and have a safe aquarium environment are all essential to a successful acclimation process.

Picking Appropriate Fish

The first thing every aquarist should do when planning a new tank is to research their chosen fish. Some fish get along well with their own species and need lots in their group to be happy, while others are loners. Some fish are just fine to be kept with other species, where others are less welcome—knowing what kind of fish others will get along with is important to preventing fights.

Additionally, it is essential to ensure that all tank inhabitants like the same kind of water. Freshwater aquarium fish originate from all over the world and want to be in the same type of water as their native home. For instance, the popular neon tetra originally comes from the Amazon River Basin. Even though the neons you can buy at your local fish store were most likely tank-bred, they’ll still be happiest in aquariums that replicate Amazon River Basin conditions.

Buying Source

Once you’ve decided what kind of new fish you’ll add to your aquarium, the next step is to find a reputable source from which to buy them. Whether buying online, from a pet store, or from a local breeder, make sure to choose a source that has a good reputation. There are lots of less than honest sellers in the world. Checking out reviews online or asking other aquarists is good practice to ensure you are getting quality fish. 

Picking Healthy Fish

The last thing an aquarist wants is to harm their new fish – this is why the acclimation process is so important. However, if you purchase unhealthy fish, acclimating them appropriately is not a guarantee that they will survive. Furthermore, adding unhealthy fish to the tank can cause your existing fish that are perfectly healthy to become sick or even die.

While there are some obvious signs of unhealthy fish, such as visible irregularities or odd behavior, the characteristics are frequently harder to spot. One way to help determine if a fish is sick or healthy is to buy in person and ask the store staff to feed the fish while you watch. Healthy fish will vigorously go after any food in the tank. Watch for the most active fish and ask for these to become your new pets.

Cycling the Tank

And of course, before adding any animals to your aquarium, make sure to cycle the tank. Tank cycling establishes healthy bacteria that are essential to the long-term success of an aquarium. Without these beneficial bacteria, there will be nothing in the tank to break down fish waste, and the fish will either die or become very stressed. Read more on how to cycle an aquarium here.



We’ve all probably had about enough of quarantining due to the pandemic, but it is the best way to start transferring fish to a tank safely. Even the most reputable fish store can have issues with unhealthy fish, so keeping any new aquarium additions in quarantine ensures it is safe to add them to the tank.

A quarantine tank doesn’t have to be fancy. After all, it is just a temporary home, but it should be of an appropriate size for all new additions to fit comfortably. The only other must-have items are a suitable water pump with filtration and a few decorations. The decorations aren’t strictly necessary, but they will help the new fish feel a little more comfortable in their short-term tank. You can learn more about setting up a quarantine tank here.

Keep the fish in quarantine for one to two weeks. During this time, observe your fish for any signs of illness. Watch for fish that seem to be gasping for breath, aren’t as active as other fish, or have visible symptoms of disease such as uneven coloring, bulging eyes, or thick, visible mucus layers. Other signs of an illness can look like dark spots on the fins and or gills.

Once you’ve observed your new fish for a week or two, you’re ready to start transitioning them to their new home.

Feed Your Existing Fish

Before taking any steps to transfer new fish, make sure any other fish you keep are happy and docile by feeding them. Being introduced to new tankmates can be stressful for your fish. Even if they’re peaceful, fish in the existing tank will often have a social hierarchy between members. This can cause stress when new fish are introduced, as they will have to figure out where the new members belong in the order. 

Additionally, new fish are often skittish and frightened when they are first added to a new tank. Feeding time can be incredibly stressful, as the fish that are already comfortable in the tank often become very active and boisterous around dinner. The added activity of feeding time can make the transition even more stressful for new fish, so it is a good idea to wait for several hours before adding food to an aquarium after transferring fish.

Regulating Conditions


One of the most significant risk factors that can cause fish to be stressed or die as a result of transferring them to a new tank is shock. If the water temperature or pH range is significantly different between the water the fish are currently in than the water they are going to, it can cause great harm. Imagine being in swim gear on a beach in Florida, then being instantly transported to an igloo in the arctic. You’d survive just fine in both places if you had the right gear and time to acclimate, but the sudden change would be a massive shock to the system. This is the same principle as adding new fish to an aquarium without acclimation.

First, check the water parameters of the water in your tank and the water the fish are currently in. If there is a significant difference in temperature, pH, or hardness, steps should be taken to make them as close as possible. Use an underwater heater to regulate temperature, and see our guide on ways to lower pH levels. If they aren’t already, put the fish into a plastic bag to start the transfer process.

Once the water parameters are as close as possible, start transferring water from the tank into the bag that holds your fish. This should be done slowly. Start by adding about ½ to one cup of water from the aquarium to the bag. Add a little more water to the bag every 20 minutes for one hour. This is sufficient time for the fish to acclimate to the new water temperature and parameters.

Next, empty water from the bag. It is essential never to add bag water into the new tank, as there could be diseases, pests, or pathogens transferred between locations through the water. Hold a net over a safe place, such as a bucket, and pour the bag water through the net. If using a sink, be sure that the drain is securely blocked. The net will then catch the fish, which are then ready to be moved to the tank.

Making the Transfer

Now that the fish have been acclimated to the new water conditions and are in the net, it is time for them to be added to the aquarium. However, it is best not to turn the net over and dump new fish in the tank. Like the example above, if you were transported from the beach to an igloo, it would be a very bewildering experience. To decrease the amount of stress and confusion, gently lower the netted fish into the tank. 

Then, allow the net to remain stationary in the water until the fish swim out on their own. It might take several minutes for the fish to orient themselves and find their way out, but that is just fine. Resist the temptation to help out your fish by waving the net in the water, as this can be even more disorienting. Remember, these are tiny creatures driven by instinct, so transferring tanks can be very stressful. Reduce the stress of moving tanks on your new pets by using gentle methods of transfer.

Adding Multiple Fish

Chances are, more than one new inhabitant is going to be added to your tank at once. So, how do you choose what order? Can more than one go in at a time? What if you want to add more than one species? A good rule of thumb is always to add the smallest, shyest fish first. This way, they will have a chance to explore their new surroundings without having to look out for bigger, more aggressive tank mates.

Whether you plan on stocking a giant aquarium or a ten-gallon tank for a child’s room, adding fish slowly is critical in a successful transition. If all your chosen fish are of the same species, add around five to eight per day. If the fish you’ve selected are of different species, add only one or two per day. It can be tempting to fill your tank with colorful, jewel-like fish as soon as possible, but adding them slowly will help them adapt better and become more comfortable with their new home.

While it may seem like a pain to make multiple trips to a store to purchase more fish, think about it as an exciting event. All fish enthusiasts love looking at fish swimming peacefully around the tank, so making repeated trips can be seen as a fun occasion and can be enjoyed by the whole family.

Observe New Fish for Problem Signs

Now that your new fish are safe and comfy in their new home, observe them closely for a few weeks to ensure their health. Even if proper quarantining methods are followed, some illnesses may still be carried over from their original tank. 

Additionally, look out for any signs of stress. Stress in new aquarium fish often shows in a fish’s behavior. If fish seem to be hiding often or swim around the tank seldomly, they may be stressed. The most common reason a fish will show these behaviors is that they don’t have enough hiding places. It may seem counterproductive to add more hiding spaces if a fish hides all the time, but this is what they need.

If a fish has plenty of options to turn tail and run when they feel threatened, they are more likely to come out in the open. Try adding more plants and decorations with holes big enough for fish to hide in.


Purchasing new fish to add to an aquarium is an exciting event. It can be tempting to want to fill your aquarium with beautiful new pets as soon as possible. However, this can cause stress to your fish and, if done, improperly can lead to stress, illness, or death. Choosing healthy fish from a reputable seller is the first step in the process. Next, quarantining new fish to watch for infection or sickness signs will keep it from spreading through the tank to other fish.

Then, ensuring the water parameters match between the fish’s existing water and their new home will reduce the risk of shock. Adding new fish slowly and gently is essential when netting them into a new tank. Once the process is over, you will have a fantastic aquarium that will provide entertainment and relaxation for years to come.

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