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  • Writer's pictureStewart Kessel

Introducing New Fish into your Aquarium

Updated: 15 hours ago

Correct acclimatisation is imperative


There is nothing like the thrill of purchasing new aquarium fish, but do you know how to introduce them correctly? If not done properly, the joy of the new purchase can quickly pass when the fish mysteriously die shortly after introduction. The immediate reaction is to typically blame the fish shop for selling weak and unhealthy fish. However, their sad demise is more often to do with transportation or the method of introduction into their new aquarium home.

Boy transporting new fish for introduction new aquarium

Commonly beginners will be advised to float the fish bag in the aquarium for a while to equalise the temperatures and then release the fish. Often this is done in such a way that it introduces all the bag contents into the aquarium. Although equalisation of the temperatures is essential, this method can still cause shock to the fish and introduce unwanted chemicals and disease from the water.


Before discussing the correct method to acclimatise your new fish, let's briefly touch on the subject of quarantine.


Quarantine aquarium

A two to three-week minimum quarantine period is advisable before introducing new fish into a community aquarium. Careful observation of the fish and their behaviour during this time will help ensure that they are healthy and there is no introduction of unwanted disease or other pathogens to their new home. If needed, any signs of an illness can be treated whilst in quarantine.


A quarantine tank will typically consist of a basic aquarium setup - with a heater and a matured filter, often a simple air-driven sponge filter. Generally, a substrate is unnecessary, and aquarium décor is kept to a minimum to enable close observation of the new fish. However, a couple of objects to allow some hiding places is advisable. Good examples would be a terracotta plant pot used as a cave or a bushy artificial plant - both of which are easy to sterilise after use.


Remember, it is equally important to acclimatise your fish when transferring them from the quarantine tank to their final home.


If you don't have a quarantine tank, you need to be extra vigilant with your purchases. You must be very selective with where you purchase your fish, ask about their quarantine regime, observe the fish over time, and avoid buying them if there are any signs of disease on the fish or any other inhabitants in the sales aquarium.


One suggestion is to observe the desired fish during multiple visits over a few weeks, maybe if possible, agreeing with the fish shop to reserve the fish, avoiding the disappointment of someone else purchasing them.


Factors to consider when acclimatising and introducing new fish purchases


Initially, let us consider what is needed to acclimatise and introduce your new fish purchases successfully.


First and foremost, it is crucial not to stress the fish. Many factors can stress fish, including loud noises, changes in their environment and transportation - see a later article on how to transport fish.


The critical focus during the acclimatisation and introduction of the new fish is water parameters. Differences in and any sudden changes in water chemistry and temperature will stress and can kill your fish. Hence, slow equalisation of the water parameters in the bag used for transportation and aquarium is necessary.


Why should this be if my local fish shop is just down the road and has the same water supply and chemistry? The simple answer is the fish will produce waste which in turn will affect the water chemistry. For more detailed information, read our article 'what happens to the water chemistry in a sealed bag when transporting fish'. It is also possible that you and the local fish shop may have different water chemistry due to how you have treated it. For example, either of you may have mixed tap water with reverse osmosis water or have hardscape rocks that may have leached chemicals into the water changing its hardness.


The second imperative is to avoid inadvertently introducing unwanted waste materials, chemicals, parasites, or diseases into your aquarium ecosystem. The fish themselves may be carriers of fish pathogens, or these could be present in the water. As suggested above, there will most likely be a build-up of fish waste during transportation. If fish are received directly from a wholesaler, the water may contain medication. Some fish can also release toxins if they become stressed. In all these scenarios, you will want to avoid introducing these harmful materials into your aquarium.


Remember you should have already researched the ideal water parameters for your new inhabitants, considered best stock level practices and ensured inter compatibility with the other inhabitants. All these factors, if not optimised, will stress your fish.


Therefore, the goal is to acclimatise your new fish to your aquariums water parameters slowly and over time, avoiding any external factors that will further stress or spook your fish, and then release them without contaminating your aquarium water.


How to properly acclimatise and introduce new aquarium fish purchases


Method 1 – the most common method for home aquariums


1. Turn off the aquarium lights.


2. Float the sealed bag in the aquarium water


3. Avoid any sudden movements or loud noises around the aquarium.


4. After approximately 5-10 minutes, open the bag and fold over the bag top, so it remains floating on the aquarium water surface.


5. Over the next 30-60 minutes, gradually add small quantities of aquarium water to the bag to equalise the water parameters. Leave 10-15 minutes between each addition.


6. Preferable: Use a small net or your damp scooped hands to transfer the fish gently from the bag into your aquarium. Alternative: Slowly tip the bag and allow the waters to mix freely, releasing the fish into the aquarium. The issue with this method is any waste components, chemicals, disease, or other pathogens in the bag water will be released into your aquarium water. Although diluted by the larger water volume, this can be a particular issue with smaller nano aquariums. Either way, as discussed above, this is not ideal.


Method 2 – often used by fish keepers with heated fish rooms and for larger fish species


1. Carefully decant the bag's contents into a suitably sized container – typically a tub, bucket, or bowl.


2. Use a small-bore tube to create a drip line from your aquarium into the container housing your new fish. Airline pipe is ideal for this operation. Use a valve to regulate the flow so that the water volume will gradually increase over 30-60 minutes. Do not leave unattended!


3. Alternatively, manually add small portions of water to the container to achieve the same result over the given time.


4. Once the water parameters in the aquarium and temporary container have equalised, net the fish, and transfer them to their new home.


5. Top up the aquarium with freshly treated water of the correct water parameters.


Author: Stewart Kessel CChem, MRSC


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