Astronotus ocellatus is a great aquarium fish to keep. They can be very personable and full of character. Some even interact with their owners. The first myth I'd like to dis-spell is the myth that oscars get lonely. They do not get lonely. They are not a schooling fish, and do not desire to be surrounded by other fish. Even so, some of you will want to add to your tank and are eventually going to ponder that one question; what fish make the best Oscar fish tank mates? The answer is largely dependent upon a few things.
Oscars are a large growing fish and mixing others with them can be complicated. In this small article I'll go over a few aspects of mixing aggressive cichlids and what is and isn't advisable.
Ding, Ding… Round 1, Texas Cichlid and Albino Oscar
The most important aspect of mixing any aggressive cichlid in an aquarium is the tank size. It has been said that if you have an aquarium large enough, you can mix any fish. The question is how large does your aquarium have to be to combine them. The minimum size tank that is universally recommended for one full grown oscar is 55 gallons (4ft). In that size tank, no other tankmates can be recommended. There is not enough territory for two fish to establish separate claims, and therefore the aggression would be excessive. In a 75 gallon tank (4ft) a few tankmates are acceptable. One to two small cichlids (between 3-5 inches) or one medium cichlid (between 6-8 inches) is acceptable. The determining factor, of course, is will the oscar accept them. Highly aggressive males are not good candidates for tankmates, even in a 75g. When you mix cichlids there will always be some level of aggression. Deciding when the aggression is out of hand is up to you. In a 125 gallon tank (6ft) your options grow drastically. The recommended stock list from site to site and person to person will also vary much more. I am of the opinion that a 6 foot tank can house 3 large cichlids (~12 inches), 5-6 medium cichlids, and between 10-15 small cichlids; all of this barring any excessive aggression or breeding.
Albino Oscar and Jack Dempsey Coexisting
I've put together a tankmate key to make matching other fish with your oscar easier. The key works like this. On the left are a list of potential Oscar tank mates by species. Each species has a star-rating. The rating is based upon the premise that the two fish are to be kept in the smallest recommendable size tank together at, or until they reach adulthood. Directly behind the rating is the minimum recommended size tank the two can be kept in.
Oscar Tank mate Key
5 stars = optimal, 4 stars = good, 3 stars = neutral, 2 less than ideal, 1=poor.
Amatitlania nigrofasciatum (Black Convict) ***** 75 Gallons
The convict really is the perfect Oscar Tank Mate. They are small as compared to the large bulky oscar and will not contribute significantly to the bioload of the aquarium. The majority of convicts are also very bold, and resilient. Not only are they aggressive enough to live with a much larger tankmate, but they are tough enough to brush off the occasional chase.
Thorichthys meeki (Firemouth) **** 75
Firemouths are similar in size to convicts so they compliment the giants size. They are not quite as bold or as aggressive as convicts. For that reason some will shy away from confrontation and hide. Still the two are compatible.
Rocio octofasciatum (Jack Dempsey) *** 100 Gallons
Dempseys can be good tankmates for oscars. They are a medium sized cichlid. Some can be extremely aggressive, and some can be very timid. Some males have also been known to reach 12 inches in length (though 8-10 is more common). This one is hit or miss.
Aequidens 'goldsaum'/'silversaum' (Green Terror) *** 75 Gallons
Description as per Rocio octofasciatum. One aspect of the Green terror that is often overlooked is their slow growth rate. Green terrors often will grow to 5-6 inches at 1/2 - 3/4 inch per month, but after that their growth really curbs. In general, Green Terrors may be too aggressive for the usually more passive Oscar. In a 6ft tank, the two species have a decent shot at getting along peacefully.
Parachromis managuensis (Jaguar cichlid) *** 125 Gallons
Jaguar cichlids have a bad reputation for being overly aggressive. Many of them are very aggressive, but some are fairly laid back. In a 6ft tank, the two species have a decent shot at getting along peacefully.
Cichlasoma salvini (Salvini's cichlid) *** 75 Gallons
Salvinis have been described as a 'reclusive' fish when they are not breeding or defending their territory. They can be excessively aggressive, but their cautious nature keeps them hidden most of the time.
Heros efasciatus (Severum) ***** 75
Severums are relatively docile fish. They will mostly keep to themselves, but won't be pushed around. Although they can reach 10-12 inches, they have a very thin profile and shouldn't overwhelm the bioload. Severums are an excellent choice as an Oscar tank mate but, with any cichlid, there are no givens.
Hypselecara temporalis (Chocolate cichlid) **** 90
Description as per Heros efasciatus. I recommend a 90g aquarium for Chocolate cichlids because they are a bit more bulky and messy. Their bioload is more significant therefore a 90g is more suitable.
Herichthys carpintis/cyanoguttatus (Green texas/Texas cichlid) **** 125
Texas cichlids are a bit more aggressive than your typical cichlid, and most likely will not submit to an oscar. Their size and aggression make them unsuitable tankmates for most cichlids in anything smaller than a 6ft tank. Therefore I recommend a 125g tank for the two to live peacefully.
Amphilophus labiatus/citrinellum (Red Devil/Midas) ** 125
Red devil/Midas are big, bulky, aggressive fish. Even in a 6ft tank, its hard to predict success with an oscar. Though it can work, mixing these two fish is a risk.
Multiple oscars *** 125 Gallons (6ft tank)
The bare minimum tank size I would recommend for two oscars is 125 gallons. Mixing oscars is really just like rolling dice, it's a gamble. Sometimes you'll have two that get along great and sometimes you'll have two that cannot live together peacefully even in a 6ft tank. The pairing process can be misleading as well. Most juvenile oscars are quite social and will often swim side by side with tankmates as if they were inseparable. However, when oscars mature they often become more fond of solitude and can become aggressive toward a former mate.
Keeping more than two oscars can be tricky. The number three is seemingly an unlucky number when it comes to these particular fish. The situation almost always plays out as two oscars teaming up and bullying the third oscar relentlessly. Naturally, one would think that the two fish that pair up are male and female, but this is not always the case. Sometimes two females team up and bully a male; sometimes two males team up and bully a female. If you have three healthy oscars living peacefully together, consider yourself lucky.
Tiger Oscar and Albino Oscar being friendly
If you don't want to deal with the possible territory/aggression problems encountered between two cichlids, then you can always turn to non-cichlid Oscar tank mates. Schooling fish do well with oscars. They add movement to an otherwise lethargic oscar tank, and provide a feeling of security for some of the more finnicky fish.
Tin Foil Barbs at a managable size
Silver Dollar (Metynnis argenteus) ***** 75 Gallons
Silver Dollars are good candidates for dither fish. Their unique body shape prevents them from being eaten even at a small size, and their thin body profiles keep their addition to the bioload of the aquarium minimal. Silver dollars find comfort in numbers so keeping them in shoals of less than 3 is not recommended. They are active swimmers and compliment the slow moving/lurching of the oscar.
Giant danios (Devario aequipinnatus) ** 75 Gallons
Giant Danios are another decent choice for dither fish. They are small, but very fast. They are also schooling fish and due to their small size should be kept in a larger shoal. I recommend no less than 6. Danios are definitely on the menu for oscars but their speed is what keeps them alive… some of the time. Don't be surprised when a few disappear here and there.
Tinfoil barbs (Barbonymus schwanenfeldii) * 180 Gallons
Tin Foil Barbs are a hazardous choice. If you decide to go with these guys, you'll need a large tank. I recommend a 6ft tank bare minimum. Tinfoil barbs can and do exceed 12 inches in length. They are also very active fast swimmers that, like the others, prefer to be kept in groups no less than 3. These fish grow fast, but when they are young, could easily be eaten by an adult oscar. Be wary of this choice in dither fish.
Other Possible Options
Tetras, and other large Barbs can also be used as active tankmates. However, you must choose these fish as Oscar tank mates with caution. Some oscars will eat anything they can fit in their mouths, and some would rather not have to chase their food. If you have the latter, then large barbs, and tetras may be considered as tankmates. Buenos aires tetras, Black skirt tetras, Congo tetras, Diamond tetras, Rosy barbs, and Gold barbs are all good choices. Again, don't be surprised if they do start disappearing.
Catfishes and plecostomus are also popular oscar tankmates. Some make excellent tankmates as they are scavengers that eat any left-over food a messy oscar may have left behind. Others are not great choices because of their large adult size. Due to the differences in body shape, catfishes are rarely looked upon as threats by most cichlids. And their unique look adds that extra bit of extraordinary to an otherwise normal aquarium.
Spotted/Four-line Pictus catfish (Pimelodus pictus/blochii) * 75 Gallons
Pictus Cats are a dangerous choice. Spotted pictus cats are probably the most readily available catfish in big chain stores. They are active swimmers and good scavengers but prefer to be kept in groups. Their relatively small size (5 inches) is what makes them a gamble with your oscar. Eating a pictus cat might be your oscars last mistake. Both Spotted and Four-line pictus cats have bony pectoral protrusions that they use for defense. Not only are they sharp at the tip of the bone, but they are also jagged on the anterior surface making it extremely difficult to remove should this weapon punture any tissue. I would say that the Four-line pictus is a safer choice as they grow to 8-9 inches in length, but still their long thin bodies make them a target for determined oscars.
Spotted/Striped Raphael catfish (Agamyxis pectinifrons/Platydoras costatus ) *** 75 Gallons
Raphaels are another option as a tankmate. Striped raphael cats can grow to 8 inches, and Spotted raphael cats grow to 5 inches roughly. Both of these cats have hard armored bodies that deter most predators from eating them, and I would say this is true for oscars as well. The downside of both Raphael cats is that they tend to be very nocturnal. They hide for the duration of the day, and really are only active when the lights are out.
Synodontis eupterus (Featherfin Syno) **** 75 Gallons
This is one the most overlooked catfish available in pet stores. They are a peaceful species that reach 8-9 inches in length. This is one of the most compatible tankmates for large cichlids as far as the common catfish is concerned.
Plecos ** 100 Gallons
The Common plecostomus has got to be one the most purchased fish in the hobby today. These fish are purchased with the mindset that they will “clean your tank”. Don't be fooled folks. Most plecostomus are more messy than your average fish. They do eat algae that grows on the sides of your aquarium, but they also create a lot of waste and add a large portion to the bioload of the tank. There are two species of plecostomus sold as “Common Plecos”: Hypostomus plecostomus, and Pterygoplichthys gibbiceps. The H. plecostomus normally max out around 12 inches, while the P. gibbiceps grow to 18-20 inches. Either way you look at it, these are large messy fish. Even though they may live peacefully with an oscar, I would avoid these fish, especially if you only want one to clean the tank. Those who choose the common pleco should be prepared to do heavy, frequent water maintanence.
The provided key's and this article as a whole is a guideline. It's meant to give you an idea of what types of fish may or may not work with your oscar. It certainly doesn't mean that other ideas cannot work. As long as you give both fish ample space and you are proactive about reducing aggression, your options are limitless.
* All photos were taken by ~Rush~
* Photo captions by Kmuda, don't blame the author
For addition discussions about Oscar Tank Mates, please review this forum thread