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Acclimatising Fish Kate Redman

Introducing your fish, or other tank inhabitants, to their new aquarium water is an important final stage to setting up your aquarium, and should not be rushed. Exposing your new animals to a sudden change in temperature, salinity, or pH could overly stress them, and possibly lead to their premature death. Therefore, it is not wise to simply drop your fish into its new home, rather, it needs to be acclimatised slowly to its future environment.


The methods described below for acclimatising fish are not only for introducing your new animal to its new home, but should also be followed for the all important initial procedure of quarantining. Placing your fish into an occupied tank could expose already established animals to unwanted pathogens. And even if there are no animals in the tank, an infected animal could introduce diseases and parasites that may attack future additions, leading to you having to dismantle, clean, and set up your tank all over again. Quarantining should be standard practice, no matter where your fish was sourced from.

Therefore, you’ll need an intermediary tank to segregate and monitor your fish’s health. Quarantining doesn’t require an elaborate tank set up [1]. Simply set up a 45 to 90 litre (10 to 20 gallon) tank with an established filtration system, a heater (ensuring the temperature is at the fish’s required level), and enough water from your future aquarium to provide adequate coverage. Using water from your future tank in your quarantine tank will help ease the final transition of your fish into its new home. However, don’t take too much water out of the donor tank in one go, as this could upset its nitrogen cycle, and may cause any animals already in it unnecessary stress.

Once the quarantine tank has been assembled, use one of the methods listed below to acclimatise your animal before placing it into the quarantine tank. Monitor them for a period of at least two weeks [2], and ensure they are eating properly, and are in good health. Then transfer them to their final home, again, using an acclimatising method listed below.


No matter the acclimatising method you chose, it's a good idea to introduce your fish to a low-lit environment [1] to minimise shock, as sudden exposure to bright light can cause severe stress or trauma [2]. Therefore, turn off any aquarium lights, and dim the lights in the room where your aquarium is set up. Additionally, keep your aquarium lights off for at least four hours after the animals are introduced into the aquarium to help them further adjust [2].

There are several methods for acclimatising your fish to its new tank [3]:

The Floating Bag Method

The Drip Method

The Bucket Method

Each coming with its own variations and tweeks, depending on which aquarium information site you visit.

The Floating Bag Method

The floating bag method goes beyond just leaving the fish, in its bag, in the tank for a few minutes. All that achieves is that it brings the water in the bag closer to the temperature of the water in your tank, and doesn’t take into account the rest of the water chemistry [3]. So once the tied-off bag has been floating in the tank for about 15 minutes [1], there is a couple of ways you can introduce the fish to its new tank water.

The first method requires you to open the top of the bag, remove approximately 25% of the water (don’t add this to your tank), and replace it with the same amount of water from your tank [3]. Place the bag in your tank, using the tank’s hood to secure the open end of the bag. Then, over the course of about an hour, add about 1 measuring cup (less if the bag is smaller) of your tank water to the bag every 10 minutes.

The other way [2] is to cut the bag open just under the metal clip, roll its top edge down one inch, creating an air pocket, and add half a cup of aquarium water to the shipping bag. Then, add another half cup of water every four minutes until the bag is full. Once the bag is full, remove it from the aquarium, discard half the water from the bag, return it to the aquarium again, and add another half cup of aquarium water to it. As before, keep adding half cups of water every four minutes until the bag is once more full.

Either way, once the process is complete, do not dump the bag water directly into your tank. Even if the fish is being placed in a quarantine tank, you’ll want to minimise the risk of introducing pathogens that were in the dealer's tank into the water. Furthermore, any elevated levels of waste ammonia, produced by the fish as it sits in the bag, could throw your tank’s nitrogen cycle out of balance, causing the fish undue stress while the cycle adjusts. Instead, use a small net to get the fish out of the bag, and place it gently into the tank. If the fish proves difficult to net, carefully pour the bag water into the net over a large bowl or plugged sink, so you can catch the fish in the net.

The Drip Method

The second method, the Drip Method, is somewhat more advanced, but is advisable for more sensitive animals such as corals, shrimp, sea stars, and wrasses [2]. It should also be noted that this is a long process, which needs monitoring throughout. You will also need airline/air pump tubing, and a clean, 14 to 23 litres (3 to 5 gallon) aquarium designated bucket (designated for aquarium use only, as you don’t want chemical residue poisoning your animals). Furthermore, if you’re acclimating both fish and invertebrates at the same, you’ll need a separate bucket, and separate length of tubing, for each.

As before, dim the environment, and leave the tied-off bag in the tank for about 15 minutes for the water temperature to equalise. Then, carefully empty the contents of the bags (including the water) into the buckets. It’s important not to expose the animals to the air. So, depending on the amount of water that’s in each bag, you may need to tilt your bucket at a 45 degree angle to ensure the animals remain submerged [2]. If, on decanting the bag’s water into the bucket, the animal can’t be kept fully submerged, securely prop the bucket into its 45 degree angle until sufficient liquid has been added from the siphoning to allow you to put it upright.

Along the airline tubing, either tie several loose knots, or use a non-metal airline control valve, to regulate water flow from the aquarium. Then run the siphon drip line from the main aquarium to each bucket, preferably securing the tubing in place with an airline holder. That done, start the siphon by sucking on the end of the tubing you'll be placing into the bucket. When water begins flowing through the tube, adjust the drip (either by tightening one of the knots, or adjusting the control valve) so you get a drip rate of about 2-4 drips per second [2]. Allow the water to slowly fill the bucket, and once the water volume has doubled, discard half, and begin the drip process again until the volume is again double. All in all, this process should take about an hour, depending on how much water was initially in the fish bags.

Once the drip process has been completed, your animals can be transferred to the aquarium. Sponges, clams, soft corals, and gorgonias should never be directly exposed to air, or have any of their fleshy parts handled [2], as this could lead to them being damaged. Therefore, carefully scoop them out of the bucket with the specimen bag, making sure they’re fully covered in water, then submerge the bag underwater in the aquarium, and gently remove the specimen from the bag. Afterwards, seal off the bag underwater by twisting the opening, and remove it from the aquarium, upon which you’ll discard the bag and its water contents. A tiny amount of the diluted water will escape into the aquarium. So long as your animal has completed quarantine, this won’t be an issue for your main tank.

The Bucket Method

The last method, the Bucket Method [3], is a variation of the floating bag method sometimes mentioned on sites. However, instead of floating the bag in the tank, you're putting it into a clean, aquarium designated bucket instead. As with the Floating Bag Method, you open the top of the bag, replace the water via a Floating Bag Method above, and once that’s done, transplant the fish into its aquarium via a net.

The advantage of this method is that it prevents the bag water from accidentally sloshing into your tank. The disadvantage is, because the bag hasn’t been acclimatised to the water temperature of your aquarium, you will likely expose your fish to stressful variations in water temperature when you deposit it in its tank. Generally speaking, water cross-contamination should be a concern. You never know what a fish could be hiding, even when coming from a reputable supplier. Quarantining your fish should be the rule, and as such, the bucket should only be used as a last resort if, for some reason, you’re not able to carry out the quarantine process. If this is the case, remember to be aware of temperature variations when adding the tank water to the bag. Placing some water from the future tank into the bucket first, and leaving the bag (with its contents of fish) to sit for about 15 minutes, could help reduce stress.

[1] How to Acclimate a Fish (http://www.wikihow.com/Acclimate-a-Fish) Accessed 11-Oct-2016

[2] New Arrival Acclimation Guide (http://www.liveaquaria.com/PIC/article.cfm?aid=157) Accessed 11-Oct-2016

[3] Acclimating Fish to Your Aquarium (http://www.fishlore.com/acclimating-tropicalfish.htm)