Hi guys,

This is an article off my web page about keeping aggressive tank mates. I keep some very aggressive fish species that are generally not suitable for keeping with oscars, but by using methods outlined below I can reduce violence quite a bit.

I'll be interested to hear your feedback.

Inappropriate psychotic tank mates for an Oscar



These are the sorts of fish I'm talking about

The fishy social dynamic of violence:

My aquarium mainly houses fish from the Amazon River and other American freshwater systems

These types of fish are large, colourful and hardy. In my experience they are also violent, territorial and predatory! You can't blame them really, in their native waters they have to live with piranhas and giant parisites! I'd be angry too!

Keeping groups of these types of fish always leads to aggression and bullying problems. Fish are often injured and stressed. Sometimes fish are murdered by their tank mates!

To reduce this problem I drew on the knowledge of people who breed very aggressive Australian grunters. From their ideas I developed a plan to regulate the social dynamic of my tank:

The biggest fish is non-violent

The Giant Gourami is far larger than any other fish in this tank, yet it is fairly placid. The other fish like to eat each other, but the Giant Gourami prefers to munch on bananna and broccoli. Because he is much bigger than the other fish he prevents any of them assuming the dominant position. This tank has a benevolant leader.

The angry fish are kept in groups

1 angry fish will attack other fish.

2 angry fish of the same species will fight each other until one dies and then the survivor will attack other fish.

4 or more angry fish of the same species will try to fight with each other, fail (because there are too many to fight) and leave other fish alone. They must also watch their own butt all the time in case they are attacked by one of their own.

In my experience this system works without fail to reduce violence. I keep all my nasties in groups of 4 or more. If violence persists then I just increase numbers.

Fish that are stupid, fast and tough make great targets for aggression

Target fish are used to keep the nasties from thinking about bullying each other over territory.

I love stupid fish, and they don't come much dumber than cypranids (like goldfish, carp and tinfoil barbs). My 3 big tinfoil barbs are glorified beautiful idiots. No matter how many times they swim into another fish's territory and get attacked they never learn. Fortunately they are also fast and tough so when they are attacked they can survive the chomp and escape quickly (to do it all over again).

The big nasties find the tinfoil barbs so annoying that they become distracted from fighting with each other. It makes it difficult for a territory to be formed when it is never respected no matter how hard it is defended!

The proof is in the pudding - evil is tamed

When I have guests come over they have no idea about the nature of the fish in my tank. They comment on how tranquil the aquarium is. I particularly enjoy the look of shock on their faces at feeding time

Firstly I certainly would not describe carp as being stupid. Young carp may be, but as they get older they become very wise, cynical and know where danger lies, ever tried catching a 30 pounder?

I think a lot has to do with how big your aquarium is at the end of the day. Keeping fish in tanks is a huge compromise, they're not really meant to be there, really, they should be swimming around in their natural environment where they don't have too much of a problem with aggression because they are not always bumping into other fish. If you're talking about the Amazon, that is one of the largest bodies of water in the world, fish do not have a problem with space.

The majority of people have tanks that are less than 100 gallons so whereas in theory your practices are successful in taming aggression, they are just not feasible for most people.

I have various cichlids at the moment in my 300 gallon tank, these include Oscars, severums, & acara. I also have Jurupari, silver dollars and a Leporinus. So I am mixing semi-aggressive fish with nonaggressive fish and in nearly 3 years I have had no problems with aggression whatsoever. I put this purely down to having such a large tank, 8' x 2.5' x 2.5', plenty of room to move around in, plenty of hiding places, everyone lives together in harmony. Now put all those fish into a 100 gallon tank and I bet you everyone would get annoyed with each other before too long.

I can't quite understand why some people insist on mixing very aggressive fish with nonaggressive fish. After all, if you went to the zoo, you wouldn't see a zebra in with the Lions, even though they live together in the wild.

Just my two cents, but very interesting article, thanks for sharing

X2 with the carp. very smart fish. i had a pond as a kid/teenager my koi would know when my cat sat on the ponds edge or if it was a another cat trying to get a meal.( my cat never touched them even when they came up to nip his tail or paw in the water ))

I think a lot of people get bad or no advice from the shop when choosing fish so choose fish by look not on aggression and don't really understand why some get away mixing them. As said given room you could mix almost anything cos they can avoid each other. I have come unstuck myself on this issue with very bad results. taming the urge to buy something you really want but know you shouldn't buy is hard and risking it a bit could pay off, but rarely works out. i think if you want a community tank it needs to be large enough. then you need certain numbers to control aggression. get the size and number wrong and you are more likely to have problems.

Do you really think I could keep lion and Zebra together Penn ))))))

marcus wrote:


maybe a lion fish and a zebra fish

OFL wrote:

i believe yes... when the lion is still a kitten and a zebra as yearling...
that is almost the same for a 2 inches oscar and 2 inches gold fish.
you can keep them together for 6 months, but not too long before the lion
could chomp the zebra

Thanks for the feedback guys


I was referring to how I do it in my tank which is 265gal - however:

The 4+ fish rule can apply at most sizes depending on what you keep. It certainly works for grunters and many cichlids. I've found it also works for many aggressive cypranids. Also worked for aggressive marines on my aquaculture farm on a much larger scale. These are fish I have had problems with and then used the 4+ rule to reduce violence:
Jaguar cichlids
Texas cichlids
Black belt cichlids
Black sharks
Giant danios (nipping)
All these fish I have kept in groups with Oscars and the Oscars have been left alone.

Also has worked in small tanks with:
Red tailed sharks
Sooty grunters
Coal grunters
Tiger barbs (stops them nipping other fish)

From your feedback I think I will add another section to the article defining the limitations of this theory i.e. not for the inexperienced.


My experience has been that most cypranids do not respect territory in an aquarium. There may well be cypranids that do but I am yet to keep one that does. I chose my barbs to disrupt territory and that is exactly what they do all day long
I definately have respect for the cypranid ability to survive - they are a very successful group of fish.


Because we love those agressive fish and don't love multiple tanks. I love the neatness of just having one big display tank (and a few utility tanks in the shed for fry and quarantine). I do mix aggressive with non-agressive and the non-agressive do not get hassled. The situation is win-win.


I love golden severums! It is hard to find good quality severums in Australia. Might have to make another trip to asia...


This has happened to me so often that I no longer ask for advice from LFS. I research the fish fully and then go to the LFS. My last mistake involved being sold a nice fish called a bumble bee fish which I later determined to be a tilapia buttikoferi! It raised hell before I got rid of it.

I'm thinking of trying them again in a group of 6

There are quite a lot of of species of fish in the cypranid family, in all fairness to you, we were just talking about carp themselves. These are not the sort of fish to keep in an aquarium anyway.

I mentioned tank size because I recently ran a poll on here and the majority of people who hjave visited our site ran tanks considerably smaller than 100 gallons.

Since I have been keeping fish I have only ever Rehomed 7 fish. A couple of catfish, a pleco, two tinfoil barbs and just recently a couple of red Oscars. One of the main reasons why I stick to the fish I've got is because I know they work together and I really don't like giving a fish a nice home to live in, and then taking it away from it a few weeks later, call me a big softy, but I don't think it's fair. Fortunately all of my fish have gone to good homes but sometimes in my area it's not always easy to find someone that will take a fish, especially if it is quite big. So I suppose I have stayed safe and omnly mixed fish that I know I'm going to be okay with. Having said that, one of my blue acara can be a right little sh1t sometimes and I can see him becoming troublesome the bigger he gets, so I will be keeping a very close eye on this little chap. Is not as if he hasn't got a lot of room, but sometimes he likes to boss a few of the other fish around, especially the jurupari that are so laid back, it's ridiculous. I run a happy home, I will not tolerate troublemakers

We have a member's article section on the home page. If you fancy writing up an extensive article then I'm more than happy to publish it under your name. Let me know what you think

Hmmm - a full article on keeping agro fish needs to have a lot of detail because of the different conditions that it may be attempted under. I'll see what I can come up with...