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Introducing New Fish to an Established Tank

Posted by on Oct 11 2021

Adding fish to a new tank is always an anxious time. Slight differences in water temperature and makeup can make all the difference when adding new fish to an existing tank setup. If you are adding new fish to an established tank and looking for advice, our guide is here to help.


What Should I Know About Adding Fish to an Established Tank?

One of the most important things to know when introducing new fish to an established tank is not to rush and not to add too many fish to the tank at one time. Adding new fish to an already existing aquarium can be a difficult business if not handled properly and the last thing you want to do is to upset the perfectly working ecosystem of your existing tank after all of your hard work. Before adding any fish to your aquarium, be sure to test the water to ensure the chlorine levels and PH are at the right level for your new fish.

Can I Put New Fish in With Old Fish?

The short answer to this is yes. You can usually add new fish to your aquarium without upsetting your fish tank environment too much. However, You should make sure to check what type of fish you are adding to your tank before introducing them. There are a few extra guidelines we have included below to make sure adding new fish to your tank goes smoothly.

Choosing the Right Type of Fish

Of course, it goes without saying that when choosing new fish for your fish tank you should make sure that they are suitable for the environment you have created in your fish tank. Fish of similar origins should always be kept together. For example, you should not mix marine fish with tropical fish as they require different environments in which to thrive. Some types of fish can also not mix, particularly if they are prone to nipping or being nipped. Always check if the type of fish you would like to add to your tank is suitable to mix before introducing them, your aquatic specialist should be able to advise on this. Feeding your existing fish just before introducing new fish should also help ensure they are not aggressive.


Take a look at our most popular tropical fish species for some ideas on which fish to pick.


How Long Should I Wait to Add More Fish to My Aquarium?

It is a good idea to wait for new fish to acclimatise to your fish tank before adding more fish into the mix. This will allow you to pick up any problems that may occur before it has the chance to affect any more fish. In general, you should aim to leave at least two weeks in between each new addition to your tank in order to allow enough time for your new fish to adjust to their new surroundings. If your new fish should suffer from any disease, this should also allow enough time for the disease to become apparent before any new fish are exposed to the illness.

Limit the Number of New Fish Added

When adding fish to an established tank, it is important to do so in stages. Never double the amount of fish in your aquarium in one go as this can affect the ecosystem of your fish tank. By altering the makeup of your fish tank too drastically, you run the risk of harming your existing fish as well as your new additions.


Make Sure Your Fish Tank is the Right Size to Add New Fish

Fish need plenty of space to roam if they are going to remain happy and healthy so it is important t consider the size of your fish tank when considering whether to add new fish to your existing tank. The type of fish you choose will also affect how big your fish tank should be to house them. Larger fish will require a bigger tank whereas if you only own a few small fish, a smaller fish tank may be enough to house them.


How to Add New Fish to an Existing Tank

When you are ready to add your new fish to the fish tank, dim the lights and take the following steps to introduce new fish to your tank. Float the plastic bag your aquatics specialist has provided to house the new fish on the top of your fish tank water and allow it to sit for around ten minutes.


Next, add a cupfull of your existing fish tank water into the open new bag and allow to float for another ten minutes. Continue adding a cup of water every ten minutes until the bag is full before finally using a net to transfer the new fish into your fish tank, being careful not to allow any water from the bag into your fish tank. Leave your fish tank lights off for the next few hours whilst your new fish explore the tank to allow them to get used to their surroundings in a calm environment.



Whether you are a first-time fish owner or seasoned pro, CD Aquatics are here to help with all of your fish tank queries. Contact our team now to answer any questions you may have.

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