The Acclimation Box: Can This Thing Really Work?

Acclimation boxes are an essential tool for adding new fish to an existing tank, but a tool which is often overlooked or disregarded. I was once a skeptic myself, and I have a feeling many reading this are far from convinced. After all, how could this little clear box do much of anything? Let’s find out!

The general idea here is to place the acclimation box into the tank, and then place the new fish into the box. One or two PVC pieces in the box provide some cover for the fish, and the box allows you to feed the fish directly at feeding time. Then after 2-3 days or so (we will explore this timing in a bit), you let the new guy or gal out to swim freely among the established tank mates. The goal here is to diffuse potential aggression while removing the risk of injuries. By giving the established fish a chance to “check out” the new guy/gal with a safety barrier, social roles can be expressed without either the established or new fish being able to physically touch (injure) each other.


Now let me be clear; this is not a miracle tool! It will not permit you to add two species of fish which are conventionally incompatible with each other! However, it is a great tool for mixing similar species which may not always get along well. Personally, I am a devoted wrasse-keeper and I religiously use an acclimation box when adding new wrasses to my system. I am also an advocate of proper quarantine (QT) and urge you to do the same prior to adding new fish to your display tank.

When choosing an acclimation box, you may find there are a lot of choices on the market today. However, think function over form when making a selection. Be mindful of the mechanism the lid uses (hinged, sliding, or other), how the box attaches to the tank (suction cups v. hanging clips), etc. I cannot say I know of any one box on the market which is universal for every tank; you will have to decide which is best suited for you. My personal favorite is the Elite Aquatics box shown at the top of this article, and the reason it is my favorite is simple: the white acrylic bottom. I have another box which has a clear bottom, and sometimes fish stress out trying to escape through the bottom. A piece of plastic starboard on the bottom of a clear box could be just as effective as the white acrylic bottom.

Using an acclimation box is simple, but the important thing about using an acclimation box is vigilant observation from the aquarist. After the new fish is placed into the box, give them some time to settle down and for the established fish to take some interest in the new arrival. I like to take a seat across the room and watch from a distance; it becomes obvious very quickly when an established fish takes issue with the new addition. Watch for fish to approach the box and flash or display towards the new guy – this is fairly normal and expected to some degree – do not panic. If a fish “sees red” towards the new one, you will witness them trying to attack/nip the new fish through the acrylic. It is this latter situation where you are witnessing signs of incompatibility and must be patient over the next several days. Take a bit of time each day to observe from a distance; you are waiting for this type of behavior to cease. Under no circumstance should you release the new addition if these clear signs of incompatibility continue. Ideally, I like to leave the new addition in the box for 2-3 days to allow enough time to observe the existing fish but not unduly prolong any stress the box may cause on the new fish. If after 1 full day (including one night) all interactions look mild, then I may release the new fish, but I typically wait at least 2 days. If there are signs of aggression after 3 days, I continue to wait (watching for the aggression levels to drop daily). If by day 5 the situation does not improve, you should have a contingency plan in place for the new addition. This plan should not involve adding the new addition unless the aggressor is removed. If a week passes with the new fish in the box and there are still high levels of aggression, it’s time to abort the effort and accept the incompatibility. It’s worthwhile to leave the QT system operational during this time period, as it offers a convenient fallback/contingency plan for the new fish or the aggressor (assuming you can catch it!).

With wrasses, I seldom have the need to continue using the box past day 3. I continue to be amazed how much of a difference the box can make. I have seen very aggressive posturing and even nipping towards the new addition the first day and then nearly complete disinterest by day 3. I do not wish to imagine what would have happened without the box!

Give an acclimation box a try! I bet you will find your next experience of adding a new fish to your system a lot less stressful for you and for the fish!

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