If you are looking for a peaceful, easygoing fish that will get along with all of your other tank mates, the Rainbow Shark is definitely not the fish for you. These fish can get along with many other freshwater fish species, but they are by nature extremely territorial and this can be overwhelming, if not dangerous, for shier, less confrontational fish species.
Rainbow Sharks have a tendency to express dominance and territoriality over a certain area of the tank. They will claim a section of the aquarium as their own, and not leave it alone. This is most commonly done with caves and rocks, where they will defend their turf against all fish who come near it.
Be very careful when choosing tank mates, as you want to choose a fish that does not closely resemble the Rainbow Shark and also does not live in the same area of the water column. You should avoid fish that look similar to the Rainbow Shark, such as the red tail shark and bala shark, as these will become frequent targets of your Rainbow Shark’s aggression.
Other bottom-dwelling fish you should avoid are cichlids and catfish. You can get away with keeping any fish that hang out toward the top or center of the water column, such as barbs, danios, gouramis, and Rainbowfish.
To help ensure compatibility, make sure the Rainbow Shark is the last fish placed in your aquarium. This will make him less likely to try to claim certain sections of the tank as “his own,” and will reduce territorial behaviors as a whole.
You should also try to reduce the overall number of Rainbow Sharks you have in a single aquarium at a time. Like many other spices, including red tail Sharks and royal grammas, Rainbow Sharks do not like living with their own kind.
In the wild, Rainbow Sharks are solitary and will stay far away from other creatures. The same rule applies within your aquarium, but tenfold, as your Rainbow Shark will not have enough space to roam and the larger Rainbow Shark will chase the smaller ones relentlessly until they eventually die.
These behaviors worsen with age, so don’t think that time will help heal the relationship issues between your fish. Juvenile Rainbow Sharks may start off on the right foot, but their relationship will quickly decline as they age.
In your tank, you will likely notice your Rainbow Sharks preferring to hang out in one or two favored spots. This is not a cause for alarm unless it’s clear that one Rainbow Shark in particular is being bullied by the others. Some Rainbow Sharks are more nocturnal than others, preferring to leave their hiding places and swim around after dark, but others may be too terrified to leave their cave or den due to the aggressive behaviors of other fish.
That being said, if you want to keep multiple Rainbow Sharks for aesthetics or for breeding purposes, it can definitely be done. Keep a group of five or more to prevent the dominant Rainbow Shark from targeting a single fish with his bullying behavior, and never keep just two Rainbow Sharks. If you must introduce multiple Rainbow Sharks, make sure they all have ample space to swim around the tank, increasing the volume of your tank exponentially as you add more fish.