Introducing New Fish to Old Fish in an Established Aquarium

introducing new fish to old fish in an aquarium

Introducing new fish to an established aquarium is a delicate process that requires meticulous planning and execution. With years of hands-on experience and a passion for sharing knowledge, I’ve crafted this comprehensive guide to ensure a seamless transition for both your existing and new aquarium fish.

  1. Research & Planning: Before bringing home any new fish, you should research their compatibility with your current stock. Consider factors such as temperament, size, and water parameter requirements. For example, if you have a peaceful community tank with Neon Tetras and Guppies, avoid introducing aggressive species like Tiger Barbs or Cichlids.
  2. Quarantine and Observation: Always set up a separate quarantine fish tank when introducing new fish to old fish. This allows you to monitor the health of the new fish and their behavior before adding them to your main aquarium. During the quarantine period, which typically lasts 2-4 weeks, keep a close eye on your new fish for any signs of disease or stress. Treat any issues promptly with appropriate medications like API Stress Coat or Seachem ParaGuard.
  3. Creating a Welcoming Environment: To help your new fish feel at home, you should create a welcoming environment in your established aquarium. Ensure there are plenty of hiding spots using decorations like driftwood, rocks, and plants. Live plants such as Java Fern, Anubias, and Amazon Sword not only provide shelter but also improve water quality.
  4. Monitoring the Introduction: After introducing your new fish, keep a close eye on the behavior of all your fish. It’s normal for some chasing or posturing to occur as the fish establish their territories. However, if you notice any signs of aggression or stress, you may need to reconsider your fish selection.

How long does it take old fish to get used to new fish?

In most cases, old fish will start to accept new tankmates within a few days to a week. However, the complete adjustment process can take several weeks to a month. Just like humans, fish have unique personalities. Some individuals may be more accepting of newcomers, while others may take longer to warm up to their new tankmates.

You might also be wondering whether all fish take the same time to acclimate. The answer is no. Peaceful species like mollies and guppies might become fast friends within a few days. On the other hand, territorial types like cichlids may require a few weeks to establish boundaries and accept newcomers.

Factors That Influence Settling In

Not all tank newbies are created equal when it comes to how quickly they acclimatize. A few key factors can speed up or slow down the getting-to-know-you process:

  • Tank Size – In a larger aquarium, new fish have more real estate to stake out their own territory and stay out of the established residents’ way. They’ll likely settle in faster than if they were dropped into an overcrowded situation.
  • Compatibility – You did your research on species temperament and picked community fish that play nice, right? Good! Incompatible or aggressive tank mates will seriously stress out the new kids and make it harder for them to feel at home.
  • Number of New Arrivals – Was it just one or two fresh faces? Or did you go for a whole classroom’s worth of new fish? Smaller numbers tend to assimilate faster than an entire rookie class.

Receiving tropical fish

It is important to be prepared to receive new shipments of tropical fish. A few simple steps will greatly reduce the chances of fish loss due to stress and disease outbreaks.

The techniques outlined in this guide are the result of many years of research in tropical fish handling, stress reduction, and disease prevention.

The all-in-all technique

Research has shown that all aquarium fish carry a variety of pathogenic bacteria, fungi, and parasites. The aquarium itself also harbors these pathogenic organisms primarily in the gravel bed.

If aquarium fish are healthy, well fed, and have proper water conditions their immune system is able to fight off these ever-present disease organisms.

Newly arrived tropical fish are stressed due to handling and possibly poor water quality in the shipping bags or containers. Therefore, newly received fish should never be mixed with fish already in the store aquariums.

Existing fish should be consolidated into a few aquariums to make room for the newly received fish. This is called the ALL IN/ALL OUT technique. Existing fish are isolated from new arrivals to insure diseases are not spread throughout all store aquariums.

It is far more economical to treat one or two aquariums for disease than every aquarium in the store.

Preparing for new fish

Before new fish arrive, consolidate compatible fish as much as possible. A large water change and a siphoning of the gravel bed is recommended.

The aquarium’s pH level should be adjusted for specific types of fish. Add an aquarium salt to help reduce stress by aiding in osmoregulation, the exchange of oxygen, carbon dioxide, and ammonia across the gill membrane.

Add 1 tablespoonful (20 g) of Aquarium Pharmaceuticals, Aquarium Salt(TM), for each 5 gallons (19 L) of aquarium water. This will aid osmoregulation and ease the initial stress from shipping.

Use stress coat as added protection

Every time fish are netted, their protective slime coating is removed. This natural slime coating helps prevent the loss of electrolytes (salts) from the gills and skin.

Tiny blood vessels (capillaries) lie just beneath the surface of the skin. If the slime coat is removed, essential salts are leached out into the aquarium water. Salt loss can lead to heart failure, muscle spasms, and increased disease susceptibility.

The slime layer also appears to “insulate” the fish from disease organisms. Stress Coat will provide a synthetic slime coat to reduce stress caused by netting and handling. STRESS COAT has been independently tested at the University of Georgia School of Veterinary Medicine and proven to reduce stress in tropical fish. STRESS COAT also contains aloe vera extract that has been proven to aid in the healing of damaged fish tissue.

Add two teaspoonsful (10 ml) of STRESS COAT to each 10 gallons (37.8 L) of aquarium water to protect fish. STRESS COAT also neutralizes chlorine in tap water.

Adjust the water temperature if necessary in the prepared aquariums. Most tropical freshwater fish require a temperature range of 78-82oF (25-28C). Goldfish prefer cooler water, ideally between 68-72oF (20-22C).

Adding new fish to the aquarium

Shipping water should never be added to the aquarium! Water in the shipping bags may contain disease organisms, ammonia, and organic pollutants and must not enter the prepared aquariums.

Pour the fish out of the bags into a net then add the fish to the prepared aquariums. There is no need to add aquarium water to the shipping bag. Fish adjust quite easily to the well oxygenated ammonia-free aquarium as opposed to the acidic, polluted water in the shipping bags.

Immediately remove dead or dying fish. In most cases dying fish will not respond to treatment. If left in the aquarium to die they could possibly infect other fish.


Never allow healthy fish to eat dead fish. This directly transmits disease organisms to healthy fish.

Observe fish closely for 48 hours after arrival. Look for the disease symptoms described later in this guide. For example, if certain species contract Ich (_Ichthyophthirius_), begin medicating as soon as the fish arrive.

Research has shown that most tropical fish carry some type of external parasites. A general anti-parasitic medication such as General Cure(TM) will help to control a variety of external parasites such as gill worms and Ichtyobodo.

Antibacterial medications should not be used unless symptoms appear. Over-use of antibacterial medications has lead to increased resistance of bacterial fish pathogens to medications.

  • Always follow the directions on medication packages. Do not underdose antibacterial medications as it only promotes resistance to the medication.
  • Do not mix two different antibacterial medications together. An antibacterial medication can be used together with an anti-parasitic medication such as General Cure but it is best to treat one disease at a time.

Your past history of shipments will dictate the medication of new arrivals. In some cases you will be able to predict that certain species “always” seem to come down with an infection. Be prepared to treat these fish when they arrive.

Use the following guide to detect the most common symptoms of parasitic and bacterial diseases. Then follow our recommended treatments to help determine the correct medication.

In some cases several medication choices are recommended to treat a particular disease.In these cases select the medications in the order they are recommended. This is because bacterial pathogens are sometimes resistant to one of the medications. If the first medication does not cure the disease it will be necessary to use the next medication listed.