An angry glare from the family goldfish might not be the result of a missed meal, but a too-humble abode. Fish in a cramped, barren space turn mean. Ornamental fishes in aquariums big and small, across the US, might be at risk, and that’s over 180 million of them.

And talk about aggressive fish–it’s known that the evolution of land animals began when the first fish jumped out of the water, and it’s still happening today: a biologist witnessed a small amphibious fish jump with apparent skill and purpose out of a small net and back into the water. Aggressive behavior among pet fish can be seen as a series of displays and attacks separated by at least a second. Displays are body signals such as flaring fins. An attack could be a nip, chase, or charge at another fish.

Biologist Ronald Oldfield made a scientifically study of how the environment of home aquariums affects the aggressive behavior of ornamental fishes. He compared the behavior of one of the most popular pet fish–Midas cichlids–in a variety of environments: within their native range in a crater lake in Nicaragua, in a large artificial stream in a zoo, and in small tanks of the sizes typically used to by pet owners. He also tested the complexity of an environment and its effects on fish within tanks. The addition of obstacles and hiding places using rocks, plants, or other similar objects can increase the complexity of the aquarium environment. And in larger tanks, fish spent less time exhibiting aggressive behavior.
In the future, Oldfield says, "This study might help us to better understand how human behavior changes when people are placed in different social environments. (For instance), violence in prisons might be linked in part to the smaller space and reduced stimuli."

But a pet fish expert we know doesn’t agree with Oldfield, and says, "I don’t think the issue is tank size or decorations. I think it highly species dependent. Discus and Angelfish are bred in barren tanks–however if a pair is placed in a large planted tank of 200 gallons, they will still kill others of their own species. Seahorses on the other hand are raised in basically barren tanks and when moved into a decorated tank, they remain non-aggressive."

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