by Tom Sarac

Purchasing new fish for your aquarium is always an exciting event – there’s nothing quite like choosing a fish from the store and watching it explore its new home in your tank for the first time. Like most things concerning proper fish care, a little planning goes a long way when acclimating new fish.

Starting with the actual purchase of the fish, make sure you return straight home afterwards: avoid shopping trips that include multiple destinations, or circumstances that involve the fish being left in your car for any amount of time. It’s crucial to get the fish from the store to your aquarium with as few delays or interruptions as possible.

It’s recommended to keep the bag containing the fish inside another, opaque bag, to avoid scaring the fish with bright, outdoor light. Try to avoid moving and jostling the bag around – transport is a stressful experience for the fish, so keeping the bag as still as possible will help to reduce stress and ensure a safer acclimation process.

How to Acclimate

  1. The aquarium lights should be turned off. Float the bag in the aquarium for approximately 15 to 20 minutes to equalize the water temperature in the bag with the aquarium.
  2. Open the bag, fold it over the top edge of the tank, and hold it in place with a plastic clip or a similar tool. Then, gently pour in an amount of water from your aquarium equal to about 1/3 of the volume of the water in the bag your new fish are in. Do this three times in total, 10 minutes apart each time. The goal is to at least have doubled the volume of water in the bag your new fish arrived in.
  3. Carefully net the fish out of the bag and release them into the tank. Dispose of the water in the transport bag, making sure as little of it as possible is introduced to the aquarium.
  4. If the new fish are the only ones in the aquarium, do not feed them for the first 24 hours.

Basic Tips

In most cases, the acclimation process is relatively straightforward. Following the basic steps outlined above and keeping the following tips in mind will help ensure your new fish are introduced to their aquarium safely.

  • Before buying new fish, always ensure they are compatible with the fish you already have. Talk to knowledgeable store staff, read about the species online, and consult your local aquarium groups for this information. Remember that fish can be incompatible for various reasons: feeding habits, aggression, activity levels, preferred tank parameters, etc.
  • Make sure to prepare your aquarium properly. Check that all aquarium hardware, such as filters, heaters and lights, are operating correctly, and that the dimensions and volume of your aquarium are suitable for the new species of fish you plan on buying.
  • Ensure the fish food you have is suitable for the new fish you’re adding to your aquarium – if it’s not, be sure to purchase the extra food you need before buying the fish.
  • Pay extra attention to the aquarium and the fish in it for a week or two after having introduced new specimens. Watch for signs of bullying or disease outbreaks: early diagnosis is key to managing a successful aquarium!

Advanced Tips

Marine fish and delicate tropical freshwater species, as well as fish that have spent considerable time in transport or have arrived with water of a very different pH compared to your aquarium, require careful acclimation. For fish that have spent long periods in transport before arrival and are suffering from ammonia accumulation in their shipping water, extra precautions are necessary and are not listed in this article: be sure to read more about this subject online to prepare yourself for this possibility.

  • For sensitive, delicate species, measure the pH and KH of the water in the bag and in your aquarium once you arrive at home. The values should be close: do not allow more than 0.2-0.3 pH units of difference between them. You may ultimately need to adjust the pH before introducing the new arrivals to their aquarium.
  • Set up a drip line – typical airline tubing will do – for marine and delicate freshwater fish such as Discus. Tie a knot in the tubing or use an airline control valve to adjust the water flow rate: a drip rate of one to two drops per second into the bag is ideal. Ensure the original bag volume has at least doubled before introducing the fish to your aquarium.
  • A couple of extra checks on the new fish being acclimated with a drip line is always advisable. It’s important to ensure the drip rate has remained constant and to monitor the behavior of the fish during the acclimation process. If you notice labored or rapid respiration, discoloration, or stressed nervous behavior in the fish, slow the drip rate for a more gradual acclimation.
  • In the case of aggressive, territorial species, such as many of the Mbuna group of cichlids from Lake Malawi, an additional step in the acclimation process is strongly recommended. In these sorts of aquariums, the addition of a new specimen can be very difficult, as the entire tank will be divided into individual territories. Therefore, just before beginning the acclimation, it’s recommended to completely change the decorative structure in the aquarium. Doing so temporarily eliminates the established territories, giving the new fish a chance to settle into the pecking order.
  • When introducing new fish into your aquarium, do not use a fish net on species that have hard piercing scales, rays of fins or bony appendages. These species of fish can easily become tangled and trapped in fish nets, which is highly stressful for them. If the fish are in a bucket during the acclimation process, use a small plastic container to scoop them up, and pour as much of the water back into the bucket as possible without exposing the fish to air. Then, gently allow the fish to swim out of the container and into their new aquarium.

For the most successful acclimations, consider a quarantine aquarium! Set up the tank with all the typical aquarium hardware (filter, heater, thermometer, etc.) and some hiding spots, such as terracotta flowerpots. Introduce new specimens to this quarantine tank first, and observe them for at least 2 weeks to ensure they are free of disease and feeding properly. Once this quarantine period is completed, the fish can be added to the main aquarium using the methods listed above. This is the single best way to avoid accidently introducing diseases and infections into your aquarium!