Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.
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 that the fish can be very aggressive under the right circumstances. 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.
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, tinfoil barbs, and even littler fish like cherry barbs can cause chaos if the tank is too small and 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” 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!
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.
Eric Dockett (author) from USA on April 19, 2019:
@Aldrien - If you had all similar species it would make sense to overcrowd a little to alleviate aggression. But when fish are from different parts of the world they tend not to get along in the same way.
Lesson learned. Always do research before purchase. I hope you can find a solution. You may want to see if you can return some of those fish to the shop. Good luck!
Aldrien Tan from Bukit Mertajam on April 18, 2019:
Ok, a flowerhorn should not have company .
Then, the oscars should not mix with the others. Crap..and the misinformed fish shop owner told me to stock up to prevent them from fighting.
So, the possibility of peaceful coexistence in the future is quite slim.
Eric Dockett (author) from USA on April 16, 2019:
@Aldrien - If you had only African cichlids (for example) you may be able to establish a pecking order in a larger tank and keep everyone alive. As it stands you have a mish mash of African, New-world, hybrid, etc all crammed in the same tank. I suspect the flowerhorn, if it stays healthy, will eventually kill or make life miserable for the other fish.
Aldrien Tan from Bukit Mertajam on April 15, 2019:
Hmm.. from your comments, it seems I shouldn't be referring to them as SEMI-aggressive anymore.
Aldrien Tan from Bukit Mertajam on April 15, 2019:
The tank would be 55Uk/67US gallon.
Yup, they're juveniles ranging between the 3 inch Flowerhorn to 4 inch Tilapia. The catfish is somewhat bigger at over 7 inches.
Each seems to be holding it's own at the moment, and I was hoping things would get calmer over time. Well..as you pointed out, it seems not.
Perhaps it's time to reconsider the offer from a friend who wishes to give away his (4x2x2)ft tank.
Thanks for the advice!
Eric Dockett (author) from USA on April 15, 2019:
Hi Aldrien: I'd say you have some very aggressive fish in there, along with some peaceful ones. Many will grow large enough for that tank all on their own. I'm guessing your 4-ft ank is a 55-gallon? Are they all currently juveniles? It seems like your tank is shaping up to be a battle zone.
I would seriously think about separating the oscars and the flowerhorn at the very least. The catfish will get big and more aggressive as it gets older.
Unless you seperate them I don't know that the nipping is going to get better. You can try adding more plants or decorations to see if that helps, but you have a bunch of aggressive mix-and-match fish together there.
Aldrien Tan from Bukit Mertajam on April 14, 2019:
Hi there, I have some semi-aggresive fishes in a 4 feet tank for a couple months. The issue is, they sometimes attack and nip each other.. but only for a short while. How do I minimize this behaviour? The fishes are Severums, couple of Oscars, walking catfish & jewel fish, a tilapia and flowerhorn.
Appreciate your advice.
Eric Dockett (author) from USA on January 04, 2019:
@jozel - You never know if a betta will get a long with any fish until they are together. These catfish require a tank 40 gallons or bigger. Hope you have the proper setup in mind for them. Good luck!
jozel tolentino on January 03, 2019:
hi! I have a male betta, he's my favorite and the only fish I've successfully kept alive. Recently, I bought a featherfin catfish. Can they get along together? Your response would be a great help!
Eric Dockett (author) from USA on October 17, 2018:
@Andrea - Sounds like you found a very acceptable solution to your problem. I love rainbow sharks. I've kept them in semi-aggressive tanks and really enjoyed them. They do come with some issues though. Glad you got your problem solved.
Andrea Orsini on October 16, 2018:
Thank you for your reply. The glofish sharks are a new addition to the line as well as the longfin tetras. The sunburst orange sharks are stunning to look at but like the other pinks and purples in the line the galactic purple shark is not very impressive. The new longfin tetras are very beautiful and if taken care of properly the fins grow out quite long and could be compared to a beta.
As for my my shark problem I was so nervous about him that I removed him last night. Moments (seconds really) later the tank atmosphere completely changed and all my babies spread out again instead of huddled together in defense mode as if they knew the danger had passed.
I went on the glofish site to see what kind of help they could offer and their answers where to either call the store I purchased him from or the vet. ( not very helpful at all) I called PetSmart and they offered to refund me my money but I didn't want to just pass the problem on to someone else so instead I am bringing him back today as a donation with the condition that they mark him as very aggressive and only sell him to someone who has experience with aggressive and very territorial fish.
They market GloFish for classrooms and children and I believe they should put a better warning about the sharks especially the adults. They also need to make it a point to teach the stores that are selling them that one shark needs to be in at least a 20-gallon tank. I thankfully have a 150 gallon because I also have 3 other glosharks but they are very docile, great tank cleaners and even school with my longfin tetras and barbs. They were very small when I purchased them which is probably why they get along so well.
Eric Dockett (author) from USA on October 16, 2018:
@Andrea - This is the first I've heard of the Glofish shark. They appear to be genetically modified versions of the rainbow shark, which are notorious for getting aggressive when there is more than one in a tank. Or, other fish they simply don't like. A bigger tank and more decorations/hiding spots might alleviate things a bit, or possibly keeping a school of 5-6 of them so the aggression is spread around. Otherwise I'm not sure there is a real answer to your issue. I never recommend two rainbow sharks in the same tank.
Andrea Orsini on October 15, 2018:
I have a 55 gallon tank with only glofish long fin tetras and a glofish shark. They were all getting along until I added a 2nd glofish shark who is aggressive. Should I remove him or give him some time to adjust. According to the glofish site I can have 2 sharks as long as the tank is bigger than 40 gallons but I constantly find myself checking to make sure he hasn't hurt anyone.
Eric Dockett (author) from USA on October 06, 2017:
Hi Anthony. That's tough to answer without knowing what fish you intend to stock in the "aggressive" tank.
Anthony Rosas on October 05, 2017:
I have a non-aggressive tank and I am getting an aggressive tank soon. Would semi-aggressive fish be better off in the aggressive tank or the non-aggressive tank or maybe a tank of their own idk what to do?
Eric Dockett (author) from USA on November 29, 2016:
@Fahmida: I'm so sorry to hear about this disaster. You do seem to have an odd mix of fish there, and some that get a little large. You said your tank wasn't big enough for the blood parrot - do you think it might be over-crowded and this may have led to the aggression?
I hope your situation improves and I'm sorry about the loss of your favorite fish.
Fahmida Mily on November 28, 2016:
I bought a pair of blood parrot yesterday.I had dollar,albino,comet and small pearl scale fishes in my tank for a long time. As my tank is not large enough for parrots, I made a hiding place for them as this fish is a little bit shy. I am thinking of buying a large tank soon. But few hours ago I went to check on the fishes and I could not believe what I saw. One of my pearl scale was floating on the water lifelessly.The parrot ate it's head and tail.The another pearl was almost dead.Those pearl scale was my favourite ones.I'm heart broken now.I never heard of parrot eating pearl scale before. They r aggressive to my other fishes too.Always trying to attack them.
Eric Dockett (author) from USA on May 25, 2016:
Hi Hailey! You can try it, but you never know now the Betta will react to the ghost shrimp (even the female). They may ignore the shrimp, or they may chase and harass them. Have a backup plan in place (like the old tank) in case things don't work out. Good luck!
hailey on May 24, 2016:
I have three bettas each in their own tank (two five gallons and one ten). One is a female and two are males, I was wondering if it would be better to move the female to my newest tank that only has ghost shrimp or one of the males. The female and one of my males are not too aggressive, but I wasn't sure.
Eric Dockett (author) from USA on May 11, 2016:
Bill, I'm not sure if you are joking or not, but it sounds like your tank is a ticking bomb. It's only a matter of time before disaster happens. Are they juveniles? I strongly suggest separating the male Bettas.
Bill on May 11, 2016:
I keep 6 bettaz in a community tank. They are all brothers nd sisters. 4 males 2 females. Now and then they flare at each other, the rest of the time they get along fine.
Eric Dockett (author) from USA on April 15, 2015:
Wow! Sounds awesome, Craig. That must be a huge tank!
Craig Martin on April 14, 2015:
I have a very unique, "semi-aggressive" tank. It's been up and running now for over a year and everyone gets along fine. 2 Clown loaches, 2 Angelicus botia, a pleco, 2chinese algae eaters, 4 corries, 1 upsidedown catfish, 5red minor tetras, 5 black skirt tetras, 5 black neons, 1 rainbow shark, 2 balas, 3 silver dollars, 3 angelfish, 3 Australian rainbowfish, 1 African leaf fish, 2 blue ram cichlids...don't think I left anybody out... very entertaining tank with very low stress! kinda happened by accident
david on November 17, 2014:
Never had a Betta I couldn't keep in a community tank, but the trick is choosing fish the Betta won't see as a threat or competition. On the other hand, you don't want to keep anything that will be tempted to snack on those long fins either. A planted 10gal with a single male betta, 5 neon tetras and an otto or two would be a perfect setup
finn on April 27, 2014:
I will be getting a betta fish thanks to this hubs info. If you are going to be geeting a fish you should read some of this information. Yes, how big and what kind of tank I'm supposed to get. Thanks
I rate this hub a thumbs up
pinappu from India on March 22, 2014:
I love those fishes. Betta is a gorgeous looking fish but it is aggressive (alas!).
Eric Dockett (author) from USA on September 01, 2013:
Hi Donny. My experience with redtails is it's best to only have one. They are fast moving and aggressive to smaller fish, and even some bigger ones, so it's smart to stock your tank with semi-aggressive fish that can hold their own.
Donny on August 31, 2013:
This is great info, I'm about to get red tailed sharks, could youlist wich fish would go with it
torrilynn on March 06, 2013:
you are welcome Eric.
Eric Dockett (author) from USA on March 06, 2013:
Glad you found it helpful torrilynn. Thanks for reading and commenting!
torrilynn on March 06, 2013:
thanks for this very informative hub
that can really help current fish owners and future
fish owners such as myself. thanks again.
Eric Dockett (author) from USA on March 05, 2013:
Thanks Eddy. Good luck with your fish if you decide to get them!
@iguidenetwork: It's really the male Betta that are aggressive toward each other. There can be aggressive behaviors between male and female, (and even female/female) but generally not so much that it prevents baby Bettas. :-)
iguidenetwork from Austin, TX on March 05, 2013:
I've had a red Beta fish (or red fighting fish) before and I had a separate bowl for it from the rest of the other fish. I wonder how they mate, especially if putting them together would, probably, spark up a fight!
I'll be on the lookout for other pet fish you've mentioned, thanks for posting.
Voted up and useful, awesome, interesting. :)
Eiddwen from Wales on March 05, 2013:
Such a great hub. At the moment for the first time i my life we have no pets!!Landlord's rules I'm afraid .However we had toyed with the idea of setting up a fish tank. Therefore this one and others by you could come in very handy.
Have a wonderful day and I vote up,across and share.