Semi-Aggressive Freshwater Fish for a Tropical Aquarium
What Is a Semi-Aggressive Fish?
You may have seen tropical fish labeled as semi-aggressive and wondered if they were smart choices for your freshwater aquarium. This label can be confusing, and not all fish earn it for the same reason.
What does semi-aggressive fish mean? It sounds like these fish are violent, but only part of the time. Like, maybe every now and then they just go berserk, but most of the time they are calm and peaceful.
Well, not exactly. When a fish is labeled semi-aggressive it usually means, under the right circumstances, the fish can be very aggressive. Very aggressive means it could attack other fish and kill them, eat other fish as food, or relentlessly chase other fish around until they die of stress.
On the other hand, under the right circumstances, semi-aggressive fish can be very docile. While we can never know exactly what a fish is going to do, we can predict with a reasonable amount of success how they’ll react in different situations due to their temperament and natural habits. This means, if we stock our tanks wisely, even a semi-aggressive species can be a very peaceful community fish.
The first step is to know the fish you plan to stock in your aquarium. In this article, we’ll take a look at many common semi-aggressive tropical fish and what makes them potentially dangerous to other inhabitants of your tank.
Stress is one of the biggest reasons aquarium fish die too soon. If you understand these fish before you stock them, you can make sure everybody gets along, and reduce the chances of stress and aggression in your aquarium.
Some fish just don’t like other fish in their space, and because of this they get the label of “semi-aggressive”. In reality, if you match them up with the right tank mates, the instances of conflict can be greatly reduced.
A few examples:
Freshwater angelfish are new-world cichlids, and cichlids tend to be aggressive fish. They may or may not get along well with others of their kind, and they may be somewhat aggressive toward small, active fish. Consider medium-sized semi-aggressive tank mates such as gouramis and slightly larger schooling fish like black-skirt tetras. Don’t overstock the tank, and make sure the water conditions are pristine.
If you decide to keep gouramis make sure you have plenty of hiding spots in your tank in case one decides to pick on the other. Sometimes they may get along fabulously, and other times the dominant fish may decide it doesn’t want the other anywhere near it. As you can imagine, this is extremely stressful for the weaker fish, and can even lead to death. Less frequently, and especially if the tank is overcrowded, gouramis may torment other fish species as well.
You’ll often see plecos labeled as semi-aggressive. These are the suckerfish who stick to the side of the tank, and they are prolific algae eaters. How mean can they be? Usually not very, but when their needs aren’t fully met they can get aggressive, especially when there is food in the water. Plecos need plenty of algae wafers as a supplement to the tank algae they’ll eat, and it’s a good idea to have a piece of driftwood in the aquarium. If you are meeting the nutritional needs of your pleco you should have no problems. If not, you may see him lashing out at other fish during feeding time.
Aka, red-tail shark. These guys are wonderful fish, but they can get tough in certain circumstances. First, be wary of having more than one per tank, as they may not tolerate each other. You’ll also want to avoid other fish that look similar, such as Bala sharks (more on them later). Finally, you never want your rainbow shark to be the bully on the block. It’s best to house him wither other semi-aggressive species, his size or larger, that he can’t push around.
Beautiful Adult Angelfish
Large Semi-Aggressive Fish
Big fish eat little fish. That’s the bottom line. Because of their big mouths and big appetites some otherwise docile fish get labeled as “semi-aggressive”. Really, they’re just looking for lunch.
As the fishkeeper, it’s your responsibility to make sure the little guys are safe from the big guys. This means picking fish that will cohabitate in your tank, rather than establishing a mini food chain.
A good example:
African Leaf Fish
Also known as Spotted Climbing Perch, Leopard Gourami and Leopard Bush Fish. This is a very peaceful fish, and one of my personal favorites. However, it’s also highly predatory and will eat anything that will fit in its mouth. It should not be kept with aggressive species, as its peaceful nature makes it an easy target. However, it may exhibit aggressive behaviors toward other anabantids, such as gouramis. Best tank mates are any peaceful fish that are large enough that it can’t eat them!
Other fish that may be dangerous for anything that will fit in their mouth include Bala sharks, red-tail barbs, and silver dollars. They are generally peaceful shoaling fish, but if they can eat a smaller fish they will.
If you have such large fish in your tank, you want to stay away from small community fish species such as neons and other small tetras, guppies, otos, ghost shrimp and small danios. Unless, of course, you are serving up an expensive dinner.
Semi-aggressive Schooling Fish
Some schooling (shoaling) fish are very active, and because they need to be in large groups they tend to get a little anxious in a tank. Even little neons can get nippy at each other when tank conditions aren’t right. Of course, neons aren’t considered semi-aggressive because they really can’t do much harm to other fish in the tank, but if they were bigger it may be a different story.
Large, fast-moving fish such as Bala sharks, tiger barbs, and tinfoil barbs, and even littler fish like cherry barbs, can cause chaos if the tank is too small and they don’t have the appropriate number of fish in their shoal. They may chase each other relentlessly, and may even pick on other species, particularly if they are smaller.
Whenever you see shoaling fish that are labeled “semi-aggressive” this is usually why. This behavior isn’t territorial or really “aggressive” at all, but instead a stress reaction to unsuitable conditions. The solution is to keep these fish in groups of at least six, preferably more, and in large enough tanks.
As you can imagine, for fish like Bala sharks and tinfoil barbs that can grow beyond twelve inches in length you will need a very large tank. For the average aquarium owner (even those with 55-gallon setups) these fish are way too large, and it’s unrealistic to expect to keep them without problems.
Most African cichlids are highly territorial and very aggressive. They should not be kept with other tropical fish species, even new-world cichlids such as angelfish or discus fish. A cichlid tank requires a great deal of research before you go out and purchase fish, and you need to know just how to set up the aquarium.
The environments of the great African rift lakes, where these cichlids come from, are vastly different than the tropical lakes and rivers most other aquarium fish live in. African cichlids are just a totally different kind of fish, and you have to know what you are doing to keep them successfully.
That said, cichlid tanks can be very beautiful and highly rewarding. African cichlids are among the most colorful freshwater aquarium fish, and they’re about as close as you can get to the vibrant colors of saltwater fish without the expense and hassle of maintaining a marine setup.
If you want to go the cichlid route, take the time to learn about them and understand which fish can and should be kept together. Realize that you’re going to need a large tank (55-gallon minimum) and you’ll need to provide a lot of places for your fish to hide.
African cichlids are beautiful and rewarding fish but can be a nightmare for aquarium owners who aren't aware of what they are getting themselves into.
Aggressive Fish that Should Be Kept Alone
Some fish are simply too aggressive to be kept with other fish. This may be because they are territorial, or it may be because of their appetite. Or, sometimes it’s a combination of both. Here are a few fish you may want to keep alone.
These guys are big, aggressive and will eat up any smaller fish they can. In very big tanks oscars can have tankmates, under certain conditions. Other large cichlids are a good choice along with other oscars. But even in the best of circumstances, your oscar may not tolerate anything else in its tank. Very often, oscars are best kept in single-specimen tanks.
Green Spotted Puffer
If you bought one of these fish because it was labeled as a semi-aggressive freshwater fish you aren’t the first to be duped. In reality, green spotted puffer care can be very challenging, These are highly aggressive fish that need to live in brackish water, usually by themselves. Sometimes puffers do okay in very large tanks with other puffers, but generally, you’re better off keeping your green spotter puffer alone. Puffers have special needs, and if you plan to keep one make sure you research proper green spotted puffer care.
Male bettas can live in community aquariums under certain conditions, but to save yourself and the betta a great deal of stress you’re usually better off keeping him alone. Bettas have a reputation as dangerous fish, but that’s not really accurate. Yes, they will fight each other to the death, but their reaction to other fish may be aggressive, or completely indifferent. If you want to keep your betta with other fish make sure you understand what’s involved in keeping a betta in a community tank. Otherwise, he should live by himself.
Keeping Your Semi-Aggressive Population Happy
If you want to keep fish that are considered semi-aggressive in your tropical aquarium you need to understand the potential issues. Always do your research, and find out why a certain fish can be aggressive at times. Make sure you stock your tank in a way that alleviates most of the issues.
You can never keep a fish tank totally stress-free, but by making smart choices you can keep aggression to a minimum. Good luck!
Still Thinking About Adding Semi-Aggressive Fish to Your Tank?
This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.