You Wouldn't Like Your Fish When They're Angry!

By Douglas Main | September 28, 2011 9:19 am

MAD FISHAnger not your fish.

Your fish are probably pissed off if you keep them in a small aquarium, suggests a study published this month in the Journal of Applied Animal Welfare Science that looked at levels of aggression in the common aquarium species Midas cichlid (Amphilophus citrinellus). Fish stored in average-sized aquariums used by most small collectors (i.e. tanks holding fewer than 100 gallons) were significantly more aggressive and violent than fish in artificial stream environments or home in their natural habitat. With 180 million or so ornamental fish in America, that’s a lot of mad fish.

Though the results may sound like common sense—no animal likes being housed in cramped conditions—this is one of the first studies to concretely measure aggressive bouts, attacks, and other behaviors indicative of the creatures’ state of mind in aquariums of varying size and complexity. These bouts ranged from fin-flaring to nips and full-fledged bites. Similar studies have found that cramped sea urchins result to cannibalism, and that great white sharks, which are very difficult to maintain in captivity for long, tend to lash out at other sharks (especially the unfortunately named “soupfin”) when confined.

While the news may give fish owners reason to think twice before plunking their bettas into mason jars, it’s not exactly practical to line your walls with aquariums or design a 1,200-gallon reef tank like this enthusiast. Luckily the picture isn’t totally grim: researchers found that when they introduced plants or complex environments like caves into aquariums, the fish became less aggressive. By making their environment slightly more complex, you can give them greater options for exploring and “hanging out.” Consider buying a larger tank or introducing more flair into your aquarium, and the “resulting natural behavior performed by the animals may make them more visible and engaging,” as one researcher observed.

Reference: Ronald G. Oldfield. Aggression and Welfare in a Common Aquarium Fish, the Midas Cichlid. Journal of Applied Animal Welfare Science, 2011; 14 (4): 340 DOI: 10.1080/10888705.2011.600664

Image: OakleyOriginals / Flickr 

  • Trevor Goulet

    Interesting that the accompanying photo is a walleye, a prized fresh-water game fish, likely never held in captivity.

  • Carmelo Fruciano

    I haven’t had the time to read the paper. However, this stuff is well known from empirical evidence to hobbyist fishkeepers. In particular, the fact that aggression is higher in smaller tanks and putting plants or other objects (supposedly to limit the visive contact between competing fish) lowers the levels of aggression. Also, the paper is on Amphilophus citrinellus, a territorial cichlid. It would be nice to see if the results are the same with other species such as Characids (which go in groups and don’t display the same territorial behaviour) or other *teoretically* more peaceful fish.

  • B2Brazil Admin

    Great piece. To buy Brazilian ornamental fish directly from the source, use (, a comprehensive directory of Brazilian companies, including ornamental fish suppliers. is Brazil-based, and focused on facilitating business transactions between Brazil and the rest of the world. Be sure to verify the origins of the fish and compliance with applicable laws.


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