How to Acclimate Fish: A Step-by-Step Guide

Acclimating new fish to your aquarium is essential to a healthy aquarium ecosystem. The excitement of bringing home new aquatic companions is unmatched, but improper acclimation can turn things southward pretty quickly. 

A big mistake many new fishkeepers make is adding a new fish to a tank immediately without transitioning (acclimating) them properly to their new home. This can be a fatal mistake for your new fish.

The acclimation process is intricate and it follows a series of well-defined steps, which we’ve defined below. The aim? To ensure a smooth transition for your new aquatic buddies to their new environment. 

Understanding Fish Acclimation

Fish acclimation is the gradual process of helping your newly acquired fish adapt to their new aquatic environment. The process is vital because sudden changes in water conditions can shock and stress the fish causing health issues and sometimes death. 

Proper acclimation mitigates these risks by allowing fish to adjust to changes in temperature, water chemistry, and other factors at a controlled pace.

Why is Fish Acclimation So Vital?

Water parameters serve fish like how air serves humans. Seemingly small issues can cause irrevocable damage immediately and even long term.

Here are the factors you should watch when acclimating your fish.


Fish react negatively to drastic temperature changes. It affects their metabolic rate, weakening their immune system and increasing their disease vulnerability. Proper acclimation ensures gradual temperature change, reducing the risk of temperature shock.

Water Parameters

Fish are sensitive to variations in water chemistry, such as the pH levels and water hardness, and abrupt adjustments can cause stress and respiratory problems, drastically shortening their lifespan. Acclimation allows fish to adjust to these changes over time, minimizing the negative impact on their well-being.

Stress Reduction

The journey from the store to your tank can be stressful for your fish. Exposure to unfamiliar sounds, smells, and surroundings can trigger stress, which makes them more susceptible to diseases. 

Acclimation provides a buffer period during which your fish can gradually get accustomed to the new surroundings, reducing stress and helping them adjust easily.

How do I Transition My Fish to a New Tank?

Before you start the acclimation process, there are a few preliminary steps you’ll need to take to ensure the well-being of both your existing aquatic inhabitants and the newcomers. 

One crucial step is to set up a quarantine tank. This tank serves as a temporary home for new fish and is a preventive measure against potential disease transmission to the established tank inhabitants.

Setting Up a Suitable Quarantine Tank

Quarantine tanks are separate tanks that mimic the conditions of your main aquarium, allowing new fish to acclimate and recover without the pressure of interacting with established tank inhabitants. You’ll need to up a quarantine tank with similar water parameters, temperature, and equipment as the main tank. 

New fish should spend at least two weeks in quarantine, during which you can observe their behavior, health, and appetite. This period allows you to identify any signs of illness before introducing the new fish to your main tank, safeguarding the overall health of your aquarium ecosystem

Of course, this step of the acclimation process is unnecessary if your fish will live by itself in the tank.

A Step-By-Step Guide to Acclimate Your Fish

To start, gather the necessary supplies, including:

  • A clean container
  • A plastic bag containing your new fish
  • A net
  • A thermometer

Ensure the quarantine tank is fully set up and running with proper filtration and temperature regulation.

  1. Float the Bag

Gently place the bag containing your new fish on the water’s surface in the tank. This allows the water in the bag’s temperature to match the tank water’s temperature gradually.

Equalize the water temperatures: Monitor the water temperature in the bag by inserting a thermometer. Wait about 15-20 minutes for the temperatures to equilibrate.

  1. Open the Bag and Roll the Top Down

After the temperatures match, carefully open the bag and roll down the top edges to create a “floating ring” on the water’s surface.

  1. Drip the Tank Water Into the Bag

Start a slow drip of tank water into the bag using a siphon or airline tubing. Adjust the drip rate to about 2-4 drops per second. This method helps acclimate the fish to the tank’s water chemistry gradually.

  1. Net the Fish and Discard Excess Water

After an hour or so, gently net the fish out of the bag and place them into the quarantine tank. Avoid introducing water from the bag into your main tank, as it might contain pollutants or pathogens.

How Long Do I Leave Fish in the Bag in the Tank?

Float the bag in the tank for about 15 minutes when acclimating the fish to your tank. That helps the fish adjust to the temperature in the tank slowly. The duration can vary, however, based on factors like fish species, temperature differences, and stress levels. 

The goal is to gradually equalize water temperatures without stressing the fish. Following the step-by-step acclimation process outlined will help you determine the appropriate time to transfer the fish from the bag to the tank.

Do You Have to Wait 24 Hours to Put Fish in a Tank?

It’s a common misconception that you must wait 24 hours before adding new fish to your tank. The logic is that waiting 24 hours allows you to test for chlorine and bacteria before introducing your new fish.

While this was commonplace in the past, it isn’t always necessary. Waiting 24 hours can be beneficial if you want to ensure stable water parameters, but it isn’t a strict rule.

Monitoring and Observing Your Fish

As you navigate the acclimation process, closely monitor your fish’s behavior and health to detect any issues early on and take appropriate action. Watch out for signs that indicate these your fish might be experiencing stress or illness:

Erratic Behavior: If your fish is darting around the tank, exhibiting rapid breathing, or gasping at the water’s surface, it is a giveaway that something might be wrong.

Clamped Fins: Fins held tightly against the body can indicate distress. Clamped fins are a typical response to changes in water conditions.

Lethargy: A fish that remains inactive hovering in one spot or hiding excessively, might be experiencing stress from the new environment.

Physical Abnormalities: Keep an eye out for visible injuries, lesions, or any unusual growths on the fish’s body. These can spell a squabble, especially if there are multiple fish in the tank.

Appropriate Actions to Take During Fish Acclimation

Should you spot any of these signs during the acclimation process, take the following actions to address the fish’s well-being:

Delayed Introduction

If you notice signs of stress or illness, consider postponing the fish’s introduction to the main tank. Allowing them more time in the quarantine tank can help them recover and adjust before joining the community. It’ll also buy you time to observe the situation.

Seek Veterinary Advice

If the symptoms persist or worsen, seeking advice from a veterinarian with expertise in aquatic animals is recommended. They can provide professional guidance and suggest potential treatments if necessary.

Transitioning Fish to the Main Tank

The moment has come to introduce your acclimated fish to their new home, the main tank. To ensure a smooth and stress-free transition, do the following:

  1. Dim the Lights

Before introducing your new fish, turn off or dim the aquarium lights. It helps reduce stress during the introduction process, calming them ahead of the already potentially stressful transition.

  1. Introduce The New Fish During Feeding Time

Timing matters. Introduce the new fish during feeding time for your existing tank inhabitants. This diversion can reduce attention towards the newcomers and facilitate a more seamless integration.

  1. Release with Care

Using a soft net, gently release the fish into the tank, preferably near a hiding spot. Allow them to swim out of the net on their own and explore their new crib.

Addressing Compatibility and Minimizing Aggression

Fish have their personalities and hierarchies, and introducing new fish can sometimes lead to territorial disputes or aggression. To minimize such issues, follow these guidelines:

Research Compatibility

Before introducing new fish, research their compatibility with the existing tank inhabitants. Some fish species are more prone to aggression than others. Knowing their behavior patterns can help you make informed choices.

🐠 Here are 5 best and worst tank mates for betta fish.

Provide Hiding Spots

Create hiding spots in your tank using decorations, plants, or structures. These hideouts offer shy or stressed fish a safe place to retreat if they feel threatened.

Rearrange Decorations

Prior to adding new fish, consider rearranging decorations in the tank. That disrupts established territories, reducing the likelihood of aggressive behavior.

Remember that some aggression is normal during the introduction phase. However, consider alternative strategies or rearrange the tank setup if the aggression becomes severe or continuous.

Troubleshooting and Common Acclimation Mistakes

Fish acclimation is a delicate process, and mistakes can happen even with the best intentions. Here are some common errors and their solutions to help you navigate the acclimation journey successfully.

Mistake 1: Skipping Quarantine

Skipping quarantine and introducing your new fish directly into the main tank is tempting, however, this can lead to disaster. Quarantine allows you to observe their health and ensures any potential diseases don’t spread to your existing tank inhabitants.

Solution: Always dedicate a quarantine tank for new arrivals. 

Mistake 2: Rapid Introduction

The temptation to rush the process is certainly relatable, but impatience can harm your fish’s health. Dumping the fish quickly into the tank is a shock to their system. Instead, stick to the step-by-step acclimation process mentioned earlier. 

Solution: Gradually acclimatize your fish to the tank so they have enough room to adapt to the new environment.

Mistake 3: Neglecting Compatibility

Adding fish without considering their compatibility can lead to territorial battles and aggression. Different species have varying temperaments, so it’s vital to research their behavior before introducing them. You can only create a harmonious community by selecting fish that get along well and share similar requirements.

Solution: Conduct adequate research to ensure that your tankmates are peaceful and compatible.

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Mistake 4: Ignoring Signs of Stress

Fish communicate their distress through physical signs such as the ones mentioned in this article. Ignoring these signs can lead to serious health issues. Take prompt action if you notice any stress indicators during acclimation or after the introduction. 

Solution: Observe your fish’s behavior carefully and respond immediately.

Mistake 5: Overcrowding

The allure of a vibrant and diverse tank can tempt you to add too many fish at once. However, overcrowding increases stress, competition for resources, and deteriorating water quality. Avoid this mistake by introducing fish in small groups and allowing the biofilter to adjust to the increased bioload.

Solution: Exercise restraint, introduce your fish in small batches, and allow enough time for adequate adjustment.

Acclimation Process: Saltwater vs. Freshwater Fish

Does the acclimation process differ for saltwater and freshwater fish?

Yes, the acclimation process can differ for saltwater and freshwater fish due to differences in water chemistry and habitat. While the basic principles of acclimation remain the same, there are some variations to consider:

Saltwater Fish Acclimation

Saltwater fish come from marine environments with specific salinity levels. When acclimating saltwater fish, you’ll need to be extra careful to gradually match the bag water’s salinity to your tank’s water. We recommend drip acclimation as it allows the fish to adjust to salinity changes over time. Saltwater fish are also sensitive to pH and temperature variations, so a slow and careful process is essential.

Freshwater Fish Acclimation

Freshwater fish are typically more adaptable to changes in water chemistry. However, they can still experience stress if introduced abruptly. The process for freshwater fish involves equalizing water temperature and gradually introducing them to the tank’s water. 

While salinity isn’t a concern, monitoring parameters like pH, hardness, and ammonia levels is crucial.

Your Fish is Aquarium Ready!

Congratulations on acquiring new fish for your aquarium! With the insights and guidelines in this article, you now know how to acclimate them properly and ensure a smooth transition. Remember, patience, attention to detail, and the well-being of your aquatic companions are key priorities.

Have you ever added new fish to an existing aquarium? How did you acclimate them?

Share your experiences and insights in the comments and gift this piece to another dedicated fishkeeper about this article. Happy fishkeeping!

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