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Angelfish in your aquarium getting angry? Try changing their water less often

If you are one of the millions of proud owners of freshwater angelfish, you might be surprised to find out how simple it can be to calm the often feisty fish: Change their water less often. The angelfish (Pterophyllum scalare) is a colorful cichlid native to the Amazon Basin, but is now found in aquariums around the world because of its popularity in the pet trade. But angelfish are fighters: A school’s social hierarchy is determined through combat, with fish using their mouths to wrestle and their tails as clubs. Angelfish also communicate social status through chemical signals contained in the urine and bile they release into the water. To see how angelfish behavior changed when those chemical signals were diluted by routine water changes, researchers replaced up to half of the water in tanks holding three angelfish. When researchers replaced a quarter of the water, wrestling and other aggressive behavior immediately increased as the fish sought to reestablish a hierarchy, they report in the current issue of Applied Animal Behavior Science. But their behavior returned to prechange levels after about an hour. When they replaced half the water, attacks ticked up for more than a day before settling down to baseline levels. And if they didn’t change any water, the angelfish carried on as normal. The results suggest that changing the water blinds angelfish to key cues that can help keep the peace—similar to how closing your eyes during a conversation might prevent you from picking up on potentially crucial body language.