Acclimating New Fish

How to acclimate new fish safely with temperature and water acclimation, the floating and drip methods, and why bag chemistry matters.

Why acclimation matters

New fish arrive in water that may differ from the destination tank in temperature and chemistry. Moving them too abruptly can cause shock, so acclimation gradually narrows those differences before the fish enters the aquarium. The two factors that matter most are temperature and water chemistry, and each is handled by a separate step so that neither changes too quickly for the fish to adjust.

Temperature acclimation

To reduce temperature shock, float the sealed bag in the aquarium for about 10 to 15 minutes so the bag water gradually approaches tank temperature. Fish shipped in breather bags, which allow gas exchange, should not be floated; instead they can be rested on the aquarium lid for about 30 to 60 minutes, because submerging a breather bag would cut off the gas exchange it relies on. This step only equalises temperature and does not yet address differences in water chemistry.

Bag chemistry

While fish sit in a closed bag, carbon dioxide from respiration builds up and lowers the pH, which keeps ammonia in a less toxic form. When the bag is opened, carbon dioxide escapes and the pH rises rapidly, making the accumulated ammonia more toxic and able to burn the gills. This is why bag water is not poured into the main tank.

Water acclimation

After temperature acclimation, pour the fish and bag water into a container. Add aquarium water to double the volume, wait about 15 minutes, double it again after about 10 minutes, and double once more after another 10 minutes. This gradually mixes the fish into the tank's water chemistry.

Drip acclimation

Drip acclimation is a more gradual alternative that uses a length of airline tubing to slowly add tank water to the container holding the fish, easing the transition for more sensitive species.

Releasing the fish

Net the fish out of the container into the aquarium rather than pouring in the bag or container water. Keeping the transport water out of the tank avoids introducing accumulated waste, and any potential pathogens from the source, into the established system. For shipped fish, the bag can be opened over a bucket, the water poured out, and the fish allowed to swim out into the tank.

Reducing stress

If the fish appear panicked during acclimation, darkening the room or covering the container with a towel can help calm them.