How to Safely Acclimate and Add New Fish to Your Aquarium
Bringing home a new fish is exciting, but the first few hours after arrival are some of the most important. Fish can experience stress from transportation, temperature changes, different water chemistry, and being introduced to unfamiliar tankmates.
Following a careful acclimation process gives your new fish the best possible start.
Magnolia Aquatics Company — Where quality comes from home.
Before Your Fish Arrives
The best acclimation starts before the shipping box or fish bag is ever opened.
Your aquarium should already be fully established, cycled, heated if required for the species, and ready for its new inhabitants.
Do not purchase fish with the intention of setting up their aquarium after they arrive.
A properly cycled aquarium should have:
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: preferably below 20–40 ppm
Stable temperature appropriate for the species
Stable pH and hardness appropriate for the species
The most important word here is stable.
Fish generally tolerate slightly imperfect but stable water conditions better than constantly changing water chemistry.
Make sure your filter is running properly, heaters are functioning, and the aquarium has adequate aeration before adding new fish.
Step 1: Inspect Your Fish Before Opening the Bag
When your fish arrives, inspect the bag before opening it.
It is completely normal for newly shipped fish to appear stressed. Their coloration may be pale, extremely dark, or simply different from what you expected.
Fish may also sit near the bottom of the bag, breathe rapidly, remain motionless, or temporarily hold their fins close to their body.
Shipping is stressful.
Do not automatically assume a fish is unhealthy simply because it does not immediately look or behave normally.
If the fish was shipped to you, follow the seller's live-arrival or DOA policy before opening the bag if there appears to be a problem.
For fish purchased from Magnolia Aquatics Company, always follow the current DOA instructions included with your order or published with the listing.
Step 2: Turn Off the Aquarium Lights
Before introducing your fish, turn the aquarium lights off.
Dim lighting reduces stress and can significantly reduce aggression from established fish.
If possible, keep the aquarium lights off for several hours after introduction.
For particularly nervous fish, leaving the lights off until the following day can be beneficial.
Normal room lighting is usually enough for you to monitor the fish.
Step 3: Temperature Acclimation
Temperature differences can cause significant stress.
For most normally shipped freshwater fish, leave the shipping bag sealed and float it in the aquarium for approximately 15–20 minutes.
This allows the water inside the bag to gradually approach the temperature of the aquarium.
Avoid floating the bag for an excessively long period.
The purpose of this step is simply to reduce the temperature difference between the shipping water and your aquarium.
Do not place the unopened bag directly under extremely bright aquarium lights.
Important: Shipped Fish Are Different From Fish You Bring Home From a Local Store
Fish that have spent many hours inside a sealed shipping bag should generally not undergo a long drip-acclimation process after the shipping bag is opened.
During transportation, fish release carbon dioxide and waste into the water.
The carbon dioxide can lower the pH of the shipping water. The lower pH temporarily makes much of the ammonia in the bag less toxic.
Once the bag is opened, carbon dioxide begins escaping.
The pH can then rise, making the ammonia already present in the shipping water more dangerous to the fish.
Because of this, keeping a shipped fish sitting in opened shipping water for an extended period can sometimes create more risk than transferring the fish relatively quickly.
For most freshwater fish that have been shipped overnight or longer, we recommend temperature acclimation followed by transferring the fish into clean aquarium water.
Step 4: Open the Bag and Transfer the Fish
After approximately 15–20 minutes of temperature acclimation, open the bag.
Do not pour the shipping water into your aquarium.
Instead, carefully remove the fish with a clean aquarium net or gently pour the contents of the bag through a fish net positioned over a clean container.
Then place the fish directly into the aquarium or quarantine tank.
Discard the shipping water.
This prevents shipping waste and potentially unwanted organisms from being introduced into your aquarium.
Handle the fish as little as possible.
Never pick up a fish with dry hands.
When Should Drip Acclimation Be Used?
Drip acclimation can still be useful in certain situations, especially when moving fish locally between established aquariums where the fish have only been in the transport container for a short period.
It can also be useful for particularly sensitive animals when water chemistry differs significantly.
However, drip acclimation is generally more appropriate for short-duration transport than for fish that have spent a long period in shipping bags.
If you are transferring a fish between tanks and want to drip acclimate it, place the fish and transport water into a clean container and slowly introduce aquarium water.
The goal is gradual adjustment to differences in pH, hardness, and total dissolved solids.
Always monitor the animal during the process.
If the fish begins showing severe distress, prolonged acclimation should not take priority over getting the animal into clean, oxygenated water.
Quarantine: The Best Way to Protect Your Aquarium
Whenever possible, new fish should be placed into a separate quarantine aquarium before being introduced into your main aquarium.
Quarantine protects both your new fish and your existing fish.
A quarantine aquarium allows you to observe the new fish for signs of disease, parasites, injuries, or abnormal behavior without exposing your entire aquarium.
A simple quarantine aquarium does not need to be complicated.
It should have an appropriately sized aquarium, established biological filtration, a heater when required, good aeration, a secure lid when appropriate, and somewhere for the fish to feel secure.
PVC pipe, aquarium-safe decorations, caves, or artificial plants can provide hiding areas while remaining easy to clean.
Many aquarists quarantine new fish for approximately 2–4 weeks, with longer observation periods used when there is reason for concern.
The important part is monitoring the fish long enough to make sure it is eating, behaving normally, and showing no signs of illness before introducing it into an established community.
Do Not Automatically Medicate Every New Fish
There are different approaches to preventative treatment.
Some aquarists routinely medicate new arrivals during quarantine. Others prefer observation and only treat when a specific problem appears.
Magnolia Aquatics Company generally recommends avoiding unnecessary medication unless you have a reason to treat.
Medications can place additional stress on fish, affect biological filtration, and may not be appropriate for every species.
Different diseases also require different treatments.
If symptoms appear, identify the likely problem before choosing medication whenever possible.
Step 5: Allow the Fish to Settle
Once your fish enters the aquarium, resist the temptation to immediately interact with it.
Give it time.
A newly introduced fish may hide for several hours or even several days.
This is especially common with:
Catfish
Plecos
Corydoras
Cichlids
Loaches
Wild-caught species
Shy community fish
Some fish may immediately begin swimming around the aquarium. Others may remain hidden.
Both behaviors can be normal.
Stress coloration is also common.
A brightly colored fish may appear dull after shipping and regain its normal coloration gradually as it becomes comfortable.
Step 6: Do Not Feed Immediately
Most healthy fish do not need to eat immediately after being introduced.
Allowing the fish time to settle can be more beneficial than immediately adding food to the aquarium.
For most fish, waiting several hours before offering food is perfectly acceptable.
For fish arriving late in the day, it is often fine to wait until the following morning.
When you do offer food, begin with a small amount.
Do not dump extra food into the aquarium because you are worried the fish is hungry.
Uneaten food quickly becomes waste and can reduce water quality.
A new fish refusing its first meal is not necessarily a reason for concern.
Some fish require a day or two before they begin eating confidently.
Step 7: Watch the Existing Fish
Introducing the new fish successfully is only half of the process.
You also need to watch the fish already living in the aquarium.
Some species are territorial and may immediately investigate or challenge newcomers.
Minor chasing can be normal while fish establish their positions within the aquarium.
Constant attacks are not normal.
A fish that cannot leave a hiding place, is repeatedly pinned into a corner, has fins being damaged, or is being continually attacked may need to be separated.
This is especially important with cichlids and other territorial species.
Having multiple caves, plants, rocks, wood, and other visual barriers can greatly reduce aggression.
A Helpful Trick for Territorial Fish
When adding new fish to a territorial aquarium, temporarily changing the decorations can help.
Moving caves, rocks, plants, driftwood, or other structures slightly changes established territories.
This can make the aquarium feel like a new environment to both the existing fish and the newcomer.
It does not guarantee compatibility, but it can help reduce the advantage established fish have over newly introduced animals.
Introducing new fish with the aquarium lights off can also help.
Never Assume Similar Size Means Compatible
Fish compatibility depends on much more than size.
Consider:
Species
Temperament
Territory requirements
Diet
Adult size
Swimming level
Water temperature
pH
Water hardness
Current tank stocking
Available hiding places
Male-to-female ratios when applicable
A fish that is peaceful as a juvenile may become highly territorial as an adult.
Always research the adult requirements of the species you are purchasing.
Your New Fish May Look Different After It Settles
Fish coloration is affected by stress, lighting, substrate, diet, social hierarchy, age, breeding condition, and water conditions.
A fish that appears pale inside a shipping bag may look completely different after several hours or several days in a healthy aquarium.
This is especially noticeable with many cichlids.
Do not panic if your new fish does not immediately display its full coloration.
Give it time to adjust.
What Is Normal After Shipping?
Some temporary behaviors can be expected following transportation.
Your fish may breathe faster than normal initially.
It may remain near the bottom.
It may hide.
It may refuse food.
It may display washed-out coloration.
It may temporarily clamp its fins.
It may remain very still.
The important question is whether the fish begins improving as it settles into clean, properly maintained water.
A gradual improvement in breathing, movement, coloration, and interest in food is a good sign.
Warning Signs to Watch For
Some symptoms deserve closer attention.
Watch for continuously worsening breathing, loss of balance, inability to remain upright, severe bloating, open sores, significant fin deterioration, fuzzy or cotton-like growths, numerous white spots, heavy mucus production, repeated flashing or scratching, extreme lethargy that does not improve, or refusing food for an extended period.
Also watch for severe aggression from tankmates.
If something appears wrong, isolating the fish in a quarantine or hospital aquarium can make diagnosis and treatment much easier.
Whenever possible, take clear photographs or video of the fish before beginning treatment.
Those images can be extremely helpful when trying to determine what is happening.
Check the Water Before Blaming the Fish
Whenever a new or established fish begins behaving abnormally, test the water.
A surprising number of fish problems begin with water quality.
Check:
Ammonia — 0 ppm
Nitrite — 0 ppm
Nitrate — preferably kept low
Temperature — correct and stable
pH — appropriate and stable
If multiple fish begin acting strangely at the same time, water quality, temperature, oxygen levels, or contamination should be among the first things investigated.
Oxygen Is Extremely Important
Newly transported fish may benefit from strong oxygenation.
Make sure there is good surface movement.
Sponge filters, air stones, hang-on-back filters, spray bars, and other filtration systems that disturb the surface can improve gas exchange.
Warm water contains less dissolved oxygen than cooler water, making aeration especially important in warm-water aquariums.
Avoid increasing temperature unnecessarily.
Adding Multiple Fish
For schooling or social species, adding the proper group size can reduce stress.
Corydoras, many tetras, rasboras, danios, and other social species generally behave more naturally when kept with appropriate groups of their own kind.
For territorial fish, however, adding several fish at once can create very different challenges.
Research the species before determining how many should be introduced together.
Adding Corydoras
Corydoras are social fish and generally do best in groups.
When introducing new Corydoras, provide clean, well-oxygenated water and a suitable substrate.
Fine sand or smooth aquarium substrate helps protect their sensitive barbels.
Do not be alarmed if newly introduced Corydoras rapidly swim around the aquarium, travel up and down the glass, or occasionally dart toward the surface.
Corydoras can gulp atmospheric air and return to the bottom.
Frequent frantic surface behavior combined with distressed breathing, however, should prompt a water-quality and oxygen check.
Adding Plecos
Plecos frequently hide after introduction.
A new pleco disappearing behind wood or inside a cave does not automatically indicate a problem.
Provide appropriate hiding locations and food for the particular species.
Many plecos appreciate driftwood, caves, vegetables, algae-based foods, sinking foods, or higher-protein foods depending upon the species.
Research your particular pleco because dietary needs vary considerably.
Adding Cichlids
Cichlids require special attention because many species establish territories and social hierarchies.
Provide visual barriers and multiple hiding areas.
Monitor new introductions closely.
Rearranging the aquarium before adding a new cichlid can sometimes reduce territorial aggression.
Size alone does not determine whether two cichlids will coexist.
Individual personality, sex, species, breeding condition, tank dimensions, and established territories all matter.
Have a backup aquarium or divider available whenever introducing potentially aggressive fish.
Never Release Aquarium Fish Into the Wild
Aquarium fish should never be released into ponds, lakes, streams, rivers, drainage systems, or other natural waterways.
Even species that appear harmless can negatively affect native ecosystems.
If you can no longer care for a fish, contact another aquarist, aquarium store, rescue organization, aquarium club, or the original seller whenever possible.
The First 24 Hours
The first day should be primarily about stability.
Keep the environment calm.
Maintain good oxygenation.
Avoid unnecessary netting or handling.
Watch for aggression.
Offer only a small amount of food when the fish is ready.
Do not make unnecessary changes to water chemistry simply because the fish appears nervous.
Shipping stress normally improves with time in clean, stable water.
The First Week
Continue watching your fish closely during the first several days.
You should gradually see more normal behavior.
The fish should begin exploring the aquarium, responding to feeding, displaying more natural coloration, and establishing normal swimming patterns.
Some shy species may require significantly longer before becoming comfortable.
Continue checking water quality, particularly if you added several fish at once.
Remember that every additional fish increases the biological load on the aquarium.
Your filtration system needs time and sufficient beneficial bacteria to process that additional waste.
One of the Biggest Mistakes: Chasing Perfect Numbers
Do not constantly adjust pH simply because a care guide says a fish's preferred pH is slightly different from your aquarium.
Sudden fluctuations can be more dangerous than a stable value that is slightly outside the textbook ideal.
Unless you are keeping a particularly sensitive species, breeding fish with specialized requirements, or working with extremely different source water, stability should generally be the priority.
Know your water.
Know your species.
Make changes slowly.
Our Recommended New-Fish Introduction Process
For most freshwater fish shipped from Magnolia Aquatics Company:
1. Have the aquarium completely ready before delivery.
2. Test your aquarium water. Ammonia and nitrite should both be zero.
3. Turn the aquarium lights off.
4. Inspect the shipping bag and fish.
5. Float the sealed bag for approximately 15–20 minutes to equalize temperature.
6. Open the bag only when you are ready to transfer the fish.
7. Net the fish and place it into the aquarium or quarantine tank.
8. Do not pour shipping water into your aquarium.
9. Keep lighting low and allow the fish to settle.
10. Wait several hours before feeding, or feed the following day if appropriate.
11. Monitor breathing, swimming, aggression, and overall behavior.
12. Maintain clean, stable, well-oxygenated water.
Every Fish Is Different
This guide covers general freshwater fish acclimation.
Some fish and aquatic animals require specialized acclimation procedures.
Always research the exact species you are purchasing.
Temperature, water chemistry, diet, group size, aggression, adult size, and aquarium requirements can vary dramatically between species.
When purchasing from Magnolia Aquatics Company, you are welcome to contact us if you have questions about the particular fish you received.
When asking for help, sending us the following information can make troubleshooting much easier:
Aquarium size, temperature, ammonia, nitrite, nitrate, pH, tankmates, how long the aquarium has been established, how long you have had the fish, and clear photographs or video of the behavior you are concerned about.
The more information we have, the easier it is to help determine what may be happening.
Give Them Time
A fish arriving at your home has just experienced a major change.
It has been caught, bagged, transported, exposed to movement and darkness, and placed into an entirely unfamiliar environment.
The best thing you can often provide is simple:
Clean water. Stable temperature. Good oxygen. Low stress. Time.
Prepare the aquarium before your fish arrives, acclimate appropriately, avoid unnecessary handling, and monitor the animal carefully.
A calm introduction gives your new fish the best opportunity to settle in and become a healthy part of your aquarium.
Magnolia Aquatics Company
Where quality comes from home.
This guide provides general aquarium husbandry information. Individual species may have different care and acclimation requirements. Always research the requirements of the specific animal you are keeping.