Certain tropical fish species are considered semi-aggressive, indicating they are territorial and will chase away or nip at others that invade their space. Since the home aquarium is a small glass box, some of these species consider the entire area their space! However, on occasion a peaceful fish can be aggressive, it’s not uncommon to have one uncharacteristically mean fish within a bunch of peaceful types. Consider it an anomaly but we’ve personally seen this and heard of many cases from our friends. There is nothing you can really do except give him back to the pet store, but we have a few tips which may help curve general aggressiveness in the home aquarium.
A single schooling fish can become aggressive. If the species is meant to live in a school and they are alone, out of fear they will instinctively try to protect themselves. Keep at least 3 of a species that school together.
Re-arrange all the ornaments in the aquarium each time you do a water change. Over time, a fish will be more and more protective of it’s territory, but re-arranging literally resets everyone’s territory making them start over and be less aggressive towards others.
Keep semi-aggressive fish together. The fish will be able to tolerate being chased since he typically behaves this way himself. With this set up, everyone will be chased and will be the chaser regulating problems so a single fish will deal with less stress since it’s all evened out through all fish.
Make sure all fish are the same size. Smaller guys are easy targets and tend to get bullied by the bigger guys. Another alternative is to have three smaller bodies for every 1 large.
Semi-Aggressive Fish Species List
Barbs – Especially tiger barbs. It’s important to note other species can be very peaceful and even be the target of fish tank bullies, while another can be mean towards everyone, even it’s own kind. Don’t necessarily shy away from barbs, but observe any new additions for mean tendencies – if you notice one it’s best to return him to the pet store.
Large Tetras – Follows the same advice stated above about barbs, considered “community” dwellers but an aggressive individual is not uncommon. Observe the new addition to check his temperament and be sure he is compatible with cohabitants.
Sharks – Red tailed sharks get very aggressive towards each other as they grow. We advise keeping only 1 per aquarium. Freshwater sharks are not nearly as aggressive as their much larger saltwater counterparts so don’t let it’s reputation divert your attention from this species, they make beautiful additions to a larger aquarium.
Large Gouramis – Especially Kissing Gouramis, these guys are quite vivacious and mean. It’s difficult to generalize this entire sub species however as a peaceful gourami is more then likely however but on average they are considered fin-nippers.
Crayfish – Look like adorable “little lobsters” usually available for sale in blue or red. Not only are they aggressive, but they will eat fish they grab with their claws. Never keep crayfish with slow moving fish, it’s an easy meal!
Large Rainbow fish – Can also be peaceful, but aggressive individuals are quite common. Many people choose to keep species specific aquarium habitats, many Rainbow fish grow large and their fast speed makes them excellent hunters for unsuspecting smaller cohabitants.