Introducing New Fish to Old Fish

Introducing New Fish to Old Fish

Introducing a new fish to an old fish is a brilliant idea. The whole process is tricky and complicated, but you can carry it out with little expertise. Firstly, you need to consider the parameters for the fish’s assimilation in a tank. These features are preparation of tank, test water parameters, no light, consult with an expert, estimate the size of fish, check behavior, hiding places, feed.

Introducing New Fish to Old Fish

Here are 17 easy easy methods to introduce new fish to old fish. Whenever you plan to add fish to an already established aquarium, you must follow these steps:

Prepare tank

Preparation of the fish tank is essential for the transfer of tiny creatures. The whole process includes maintenance of ph. and other parameters.

Maintenance of the hiding places and decor make the tank worthy for the transportation of brand new fish. Few fish die when you transfer them from the dealer’s tank to the old fish tank. It is due to sudden shocks of change.

All of the parameters depend on the nature of the fish. Some tiny creatures like to live in changeable water conditions, but few of them as less tolerating capacity.

The decoration of the fish tank plays a vital role in the lifespan and health of the invertebrates. Never add harmful things because the new fish swim a lot. They can harm themselves by striking the rocks and driftwoods. 

Choose everything for the tank wisely because it affects the fish’s life. Also, water is the source of oxygen and feed for the fish. Adjust all the parameters in their accurate ranges. 

Test water parameters

Always check the quality of water inside the aquarium; it is one of the significant features. The quality of water directly affects the life and health of the fish.

Multiple factors determine the quality of water. The parameters include pH of water, temperature, bioload, alkalinity, nitrite, nitrates, ammonia, and hardness of the water.

Parameters Ranges
pH 7.5-8.5
Temperature 75-80 degree Fahrenheit
Bio load Low
Alkalinity 70-200 mg/L
Nitrites NIL
Nitrates NIL
Hardness of water Low
Ammonia 0 part per million

The pH of water

The unstable pH gives a shock to the new fish inside an aquarium. The acidic and base balance is necessary for the life of a tiny creature.

Always check the pH of water in the home tank. People transport the minute thing from their store to the old aquarium in a bag.

The pH changes inside the bag, and this modification changes the acidity and alkalinity. It is essential to check the ph. of the bag’s water. 

  • Maintain the home tank’s ph. in terms of the other water.
  • Time also impacts the change of ph inside the bag that is a temporary house for the new fish. It can reside inside the bag for approximately 24 hours. 
  • The fish respiration occurs in the bag, and it leads to the dropping of ph. 
  • You can add the new fish inside the old fish tank with a ph. the difference of 0-3-0.4.
  • It is an ideal situation for the new fish for excellent survival. 
  • Keep adding the water inside the bag until it matches the ph. of the old fish tank. The new fish always require a lower range of parameters than many other invertebrates. 
  • Maintain the acidity and alkalinity by adding buffers in water. Modify the ranges and stable the tank for the best survival of the invertebrates. 


The temperature of the old tank’s water directly affects the fish. Maintain it to the limits so that survival becomes easy. The fish can tolerate wide varieties of temperatures but keep it in a good state. 

Bio load

It accumulates in the fish tank due to the addition of new fish. They generate multiple algae on the sides and corners of the fish tank. Adding more plants in the tank reduces this problem. You can add algae eaters and de-chlorination of water is also a significant choice. 


It is a remarkable parameter of water that affects the ph. of water. The alkalinity below its range makes significant water changes. It helps in the prevention of shock due to water change. The fish feel healthier and alive in the low limits of alkaline. 

Nitrite and Nitrates

These are a toxic substance that occurs in the fish tank. Maintain the parameters in the lower limits. The harmful substances can be lethal for the fish, and it is not a suitable condition for the owner. 


A higher level of ammonia disturbs the respiration process of the fish. Maintain the levels of ammonia to their lowest limits.

Hardness of water

Fish cannot survive in resistant waters, and you should always keep them soft by modifying nature. 

No light

Make sure the artificial light sources are off inside the tank. It is beneficial for the life of new addition. The whole environment provides them a sense of protection all the time, and they start feeling comfortable in the new house tank. 

The old fish do not recognize the new fish in the absence of light. Also, none of them can attack each other in the fish tank. 

Consult with expert

Consult with the expert or veterinary doctor when the fish goes through the disease phase. Check the behavior of your fish in the quarantine tank. Never add the infectious diseased fish in the old fish tank.

The disease prevails to the old fish, and the whole tank becomes miserable. The professionals treat the fish and also guide you about adding them to the established tanks. 

Estimate size of fish

It is an essential parameter for the addition of fish in the aquarium. You can add multiple small fish in an old community tank.

If the new species is large, then add one to two fish at one time. You can repeat the adding process after every three to four weeks.

Never add all of them simultaneously. In the circumstances when you are planning to add multiple fish, use different quarantine tanks for them. Keep the water conditions the same in all the quarantine tanks. It helps in better survival and additional health properties for both fish.

If new fish is aggressive in nature

Check the behavior of your new fish. There are many fish that are aggressive naturally. Adding the creature in the community tank is not a good idea without eradicating their flaws. 

In the situations when the new fish is aggressive then, 

  • Separate the fish by using a net. It helps in the safety of other fish.
  • Rearrange all the decorations inside the fish tank. Sometimes the decor leads to attacking features in the fish.
  • Add more hiding places in the fish tank so that other fish can protect themselves from aggressive fish.
  • Isolate the bullying fish in another tank.
  • Always use larger tanks for the transportation process.
  • Adding the fish in small groups is better than adding large numbers. It increases the competition for food and other things, and the fish becomes aggressive. 

Hiding places

Check in the old tank that there are enough hiding places like plants and rocks. The decorative material like driftwoods, artificial items also work as a hiding place.

These spots are essential because the new fish find places to hide until they become comfortable in the tank. 

After the transportation, add an extra amount of feed for the new fish. The lack of feed leads to attack and prey.

Method to add new fish to an established aquarium

Quarantine new fish

Isolate the fish in the tank for approximately 3-4 weeks. The period of 7-8 weeks is ideal for the tiny creature. Check the parameters of water and maintain them accordingly.

In case you do not have another tank, then add the new fish in a bag. The tiny bag works as a home for the junior fish before meeting the old fish. The quarantine tank helps to recognize any disease in the new fish. 

Keep fish in the container

When someone buys a new pet, then the dealer gives them a bag or tank. This tank is known as a dealer tank by having the old water. Keep the fish in this tank for more than three weeks when you do not have another container. It is not suitable for the life of fish, but it is one option. 

Use a sealed bag

Always use a sealed bag when you put a fish inside it. Add a little amount of feed in the bag, and it keeps the entire creature busy. Never let the bag open because some furious fish ump out of it. 

Add water 

In the old bag of the new fish, add a cup of water from the new aquarium. Zip the plastic bag once again and allow the fish to float. Add half a cup of water in the bag and wait for at least 13-15 minutes.

Repeat the process for making the fish habitual in the new environment. The junior fish learn the qualities and parameters of the new tank.

This step is essential for the well-being of the tiny fish. The whole process is slow, but it helps in the stabilization of temperature and other parameters. It gives an excellent survival capacity to the new fish. 

Allow floating

The floating of fish inside the bag is necessary. DO this 3 to 4 times and keep on adding the water. 

Open the seal

In this step, unlock the bag by removing the rubber band or zip. Open the bag gradually but do not allow the transfer of water inside it. Put the bag inside the aquarium so that the water can support the fish.

Start rolling the top of the bag and repeat the process three to five times. The air starts trapping inside the bag’s rolls.

The plastic can entrap the air, and it is suitable for the fish. The plastic bag begins to float inside the tank without falling. Allow it to roll more than three times. 

Let the lights off

The light gives a sudden shock to the new creatures, and it is lethal for them. When you bring a new pet to your home, turn off the lights.

When people do not turn off lights, then the fish die in the plastic bags. The process stops at a stage when you are near to add it inside the tank.

Adding fish 

Add the fish inside the old tank with old fish, but keep the process slow. Never add the water of the old fish bag in the fish tank. It leads to the origination of itching and diseases in the whole community. 

Use the net to carry 

Use a net to carry the fish from the old bag. In this way, you can prevent the transportation of new water in the old fish tank. Never reuse or leave the net in the aquarium, and it is lethal to the fish. 

Discard net in the sink

Disposal of the net is essential after the fish transportation inside the old tank. Discard it in the sink and make sure nobody touches it. 

Add food

Add a small quantity of natural commercial feed to the aquarium. It decreases the competition between the old and new fish.

The junior fish rely on this feed, and the competition becomes low. The senior pet does not harm the tiny fish in the habitat. 

Tips to help new fish to live with old fish

Always avoid sudden trauma of pH change.

Remove extra amounts of ammonia from the fish tank for the health of all the fish community. 

Keep observing the health of your fish and consult a veterinary doctor in case of emergency.

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