kind, and they may be somewhat aggressive toward small, active fish.

Small nest-holding male fish have more aggressive behavior than large ones, finds a new study.
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I’ve already noted recommended sizes and species for knifefish tankmates in community setups. As a knifefish grows, some of these smaller community fish may need to be swapped out for larger fish so that they do not become live food. Avoid aggressive fish, such as tiger barbs and some cichlids, which may harass and thus cause stress.
Small Aggressive Aquarium Fish
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The Rainbow Shark is a freshwater cyprinid that comes from Thailand and may not be a good choice for a community tank. The Rainbow Shark likes to stake out their own territory in the tank. This territory can be in the form of small caves, rocks and even plants. They will become aggressive with smaller fish that invade this territory. Only keep one Rainbow Shark in your tank because they will not tolerate another Rainbow or Red Tail Sharks in the same tank. They may exist together for awhile, but one will end up chasing the other relentlessly until the other succumbs. You may also see an albino rainbow shark variety that is sometimes available at your local fish store. Male Betta fish are very common small fish that behave quite aggressively towards each other and will fight one another to death.
Photo provided by FlickrStudy Finds Fish in Small Boring Tanks More Aggressive | The Mary Sue
Photo provided by FlickrSmall Aggressive Marine Tank? | Saltwaterfish Forum
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I'll get straight to the point, is your aquarium big enough for one Oscar, plus some tankmates? You should already know that 55 gallons is the absolute minimum for one adult Oscar. If your aquarium does not meet this requirement then you have two choices, either stop reading now and enjoy what you've already got, or upgrade your aquarium by at least 75% and then come back and read the rest of the page. Sorry if that sounds a little bit harsh, but that really is your only two options. Most of the fish I am going to recommend as tank mates will require a minimum of 50 gallons just for themselves, not including the Oscar.Remember that you can't put any fish in with an Oscar, you've got to choose fish that will be able to live in harmony with your Oscar. First and foremost, you cannot put small community fish in with an Oscar fish because they will just be an easy source of food. I would advise not adding anything less than 5 inches to an aquarium that contains an adult Oscar fish. The type of fish that you add to an aquarium is also important. For instance, we wouldn't recommend angelfish as a tank mate because they are quite delicate with their long flowing fins, plus they actually prefer the taller type aquariums. Tankmates don't necessarily have to be semi-aggressive themselves, shoaling fish such as Silver Dollars make an excellent tankmate because they are on the move all the time and feel safe amongst numbers. Some species of bottom-dwelling fish are sometimes a good alternative because they don't really come into contact with Oscars who tend to stick to the mid-or top layers of the water most of the time. Some smaller species of cichlids make very good tankmates because even though they are quite small, they still have the ability to stand up for themselves.Also, when it comes to stocking any sunfish, it is very important to ensure that you are in fact stocking the sunfish species that you desire. Most sunfish have similar characteristics, sometimes making identification difficult. However, many are quite different in habitat usage and temperament. Therefore, haphazardly stocking sunfish into your pond can create many headaches in the future. Take the green sunfish for example. Superficially, they resemble a bluegill. However, if a sizable population of green sunfish becomes established in your pond, you can usually expect small fish size and very aggressive fish that bite swimmers. This fish can become aggressive with other, smaller fish in your tank that invade its territory. They will fight with the Red Tail Shark. Provide plenty of hiding places (caves, rocks and plants)Oldfield found that the fish, which were all juvenile so as to rule out the behavioral changes associated with mating, behaved dramatically different in the smaller tanks. In such tanks, he observed fin-flaring and other signs of moderate aggression. Sometimes, this escalated to nipping, and even attacking other fish in the tank.