The spotlight falls on an outstanding performer in fishkeeping circles. Nathan Hill undoes the envelope to reveals his nomination for best leading role.
For many in the hobby it’s the 'coming of age' fish that signifies devotion and enthusiasm. For others it’s the nightmare that eats everything in their community tank and then demands a bigger home!
However you view them, Oscars are amazing. With almost canine-like recognition of their owners, the brass to tackle a hand invading their aquarium and usually an eye on their own interior design, these are classic character fish that everyone with the appropriate facilities should dabble with at least once.
They’re surprisingly hardy, which helps explain their popularity, and, compared to some other big fish, they’re more reserved in overall size. In a small tank, this fish will soon outgrow and cost you more money — but it won’t do so as might a Pacu!
Oscars are South American and comprise two species. The common Oscar is Astronotus ocellatus, but there’s also the rarely seen Astronotus crassipinnis, a slightly smaller, darker fish. Given their trade scarcity, you could be waiting a long time if you want one.
Wild Oscars cover a large range, including Venezuela, Peru, Colombia, Ecuador, Uruguay, Paraguay, Brazil, Bolivia and French Guiana. They’ve also been introduced around the world and feral populations can be found as far as Australia.
In China and America they’re becoming an ecological hazard, as any big fish with a big appetite might. The range of wild environments they can tolerate is astounding, for as well as being reported captured all over North America they’ve been found prior to winter in Alaskan waterways. Lethality doesn’t occur until somewhere around 13°C/55°F.
They can tackle terrible water types too. Naturally, they inhabit slow-moving waterways and they’re a white water species, but will happily head into ponds, lakes, oxbows, swamps and seasonally into flooded forest and floodplains.
They have great resilience to low oxygen conditions. Older fish are more tolerant than younger ones, but adults have been kept in anoxic conditions (lacking in oxygen) for between four and six hours until they hit real trouble.
Anoxia tolerance is pretty handy for a fish that doesn’t want to gasp at the surface, exposing itself to predators. At the same time, it’s a useful trait if waiting for less tolerant fish to head up to breathe. Just sit and wait for your meals to present themselves!
This tolerance has helped domestic fish in cases of filter failures and power cuts — but none of this is justification enough to keep Oscars in tanks with low oxygen or poor circulation.
One adult has been recorded at just over 45cm/18” and size needs to be considered. Most aquarium residents will hit between 25-30cm/10-12”, which makes them tricky to manage.
A lot of keepers start them off in 120 x 45 x 45cm/4 x 1.5 x 1.5’ tanks and eventually upsize to something like 1.8m x 60 x 60cm/6 x 2 x 2’. However, some people insist on keeping Oscars in smaller tanks, maybe just 90cm/3’, but there we’ll find 'runty' fish with squat bodies, curved backs or other unsightly deformities.
Many buyers are enticed by juvenile Oscars. At this stage the fish are simply divine, showing marbled patterning and big, playful puppy-like eyes. At 4-5cm/1.4-2” they are darlings.
Sometimes staff at a particularly inept store will tell a customer that Oscars ‘are a community fish’ or that 'they only grow to the size of the tank'. Ignore them!
As they grow, almost anything that fits in the mouth becomes fair game. They also grow rather fast, especially on the high protein, nutrient-rich dried and frozen foods offered by caring fishkeepers.
Expect a growth rate of up to 2.5cm/1” a month for the first six months, slowing thereafter. Inside a year 25cm/10” is easily attainable, given enough growing space and bodybuilding diet.
Wild and captive
A captive-farmed Oscar is far removed from a wild type. Pending where captured, wild specimens can sport varying shades of olive green colour to having hints of red across the flanks.
By contrast, captive-farmed types usually have far more elaborate and striking colours. Examples are Tigar oscars, Red, Albino tiger, Albino red, Lutino, Sunshine, Golden, Super red, Blueberry and Strawberry markings. There are also the usual trappings of the over- farmed variants — namely long fin and grotesque balloons.
Worse still, given the size of the flanks and the ‘blank canvas’ nature of Albino fish — not to mention tolerance to trauma — there are hideously dyed varieties with patterns or colours injected into their flanks, à la tattoo. These latter fish should be avoided by anyone with a conscience.
Wild fish have a fascinating body marking, which explains the origins of their scientific names. Ocellatus translates as 'eye spot'and if you see the tail end of a quality wild Oscar you’ll see a very clear dark spot ringed with yellow or orange.
There’s evidence to suspect that these eye spots are an evolutionary response to the fin-eating fish, such as many piranha that inhabit the same waters as Astronotus.
In fact, comparing collections of fish with and without eye spots the fin damage to those lacking this feature was overwhelmingly different. It appears that when faced with an eye-spotted Oscar, a piranha is uncertain which end is the edible one.
Every so often a real wild rarity will appear — an Oscar that has double eye spots on each side of the tail. These account for about 0.5% of Oscars in the wild.
Oscars are gluttons and one of our great failings is to overfeed them. In the wild they are facultative or opportunistic feeders, depending on season and food availability.
Primarily they are piscivores and sometimes they play possum, pretending to be dead on the substrate, awaiting curious small fish to swim over. Once one gets near enough – wham!
Once an Oscar opens its mouth the negative pressure in the buccal cavity sucks prey in whole, and a mixture of oral and pharyngeal dentition then grind it down.
Once food becomes scarce they switch to a more mollusk and fruit-heavy diet. Snails are crushed, insects nabbed from the surface and fruits and berries pounced on.
Oscars have superb vision and can pinpoint noise direction with their hearing, so we can assume that they actually hear falling fruits and struggling bugs thrashing on the surface of the water.
In the aquarium they need a varied diet with carnivore leanings. Many good dry predator foods are available, but be aware of the vast protein levels some contain.
Unless wanting stumpy, fattened fish, foods with more than 40% protein may not be best offered.
Frozen and fresh foods are always adored, especially earthworms. Ignore the usual bloodworm and Daphnia for larger Oscars as these are too messy for them. Instead, visit your local fish counter for snippets of fresh, white fish and think frozen lancefish, cockles, mussels and prawns.
Get fresh fruit and veg too as pieces of apple, berries, tomato chunks and especially fresh peas are enjoyed.
Try to offer a mix of 85-90% meaty foods and the remainder fresh greenery or fruit.
Twice-daily feeds are always best, rather than one large, single meal. Don’t overdo volume though, as Oscars will gorge if they can and the waste can overwhelm an already struggling filter.
Once you have a large enough tank, layout is easy. A browse online will show Oscars in some of the most garish and brightly-coloured tanks out there, and with ‘unusual’ décor choices too.
Requirements are minimal. A few open caves are appreciated, as is some overhead cover in the form of large floating plants — which they will eventually destroy. Plants, aside substantial plastic ones, will be dismembered out of what appears to be sheer malice.
Substrate choices mean little to Astronotus. Don’t spend hours painfully aquascaping and sloping substrates as, whether sand, gravel or other, Oscars will rearrange it, digging huge pits and piling it all at one end of the tank.
Filtration needs to be huge. Fluval’s FX5 model comes in to its own here, but whatever model you choose expect to clean it regularly as Oscar solid waste will quickly clog your filter foams.
Heaters should be outside the tank. Oscars often take umbrage to an interior heater thermostat and will try to move it, sometimes breaking it. If opting for an in-tank heater ensure it has a sturdy guard.
Lighting is neither here nor there, though duller conditions seem to result in happier fish. Bright tanks don’t seem to faze them, but make sure the tank is covered. Not only are Oscars superb jumpers but may launch at anything potentially edible. Many curious cats and aquarist hands bear the scars…
If going for a natural tank feel, then sand, leaf litter and fallen branches are the way forward. As for tank mates; Silver dollars, large loricariids and, in some cases, other large cichlids are fine.
If not going region specific, Oscars are okay with most large tank mates, from spiny eels to stingrays, Arowana to Tinfoil barb. If it won’t fit in the mouth, and is fast or sturdy enough to cope with curious Oscars, then it will be a fine companion.
The numbers game
As they age Oscars become less keen on their own species’ company. In a 120cm/4’ tank a pair is wise and only consider multiple pairs if a 180cm/6’ or larger can be used. That said, some happily keep several individuals together in a 120cm tank.
Farmed fish seem more docile than their wild counterparts and being housed in large numbers from an early age may have some behavioural conditioning effect.
However, if you’ve had a solo fish for some time, don’t then try to add another similar sized Oscar. This is usually a recipe for disaster, as the established fish will turn on the newcomer.
Scientific name: Astronotus ocellatus.
Origin: South America, Amazonia.
Size: 45cm/18”wild, 30cm/12” aquarium adults.
Temperature: 22-25°C/72-77°F, but will tolerate much lower, although this adversely affects their metabolism and health.
Water chemistry: Aim for pH of 6.0-7.5, hardness between 5-20°DH.
Diet: Omnivore with carnivore tendencies.
Tank size: 120cm/4’ length for sub adult, 180cm/6’ for adult.
Ease of keeping: Easy. This is a hardy and tolerant fish.
Community or species tank? Non-community in the traditional sense, but gets on fine with other big fish.
Armed with the smell of fear
Many fish communicate via smell, releasing odours that act as a warning to other fish. However, Oscars carry a 'predator' smell that elicits a fright response in many prey.
The source appears to be found in the mucous layer of the fish, as well as impregnating itself into the smell of the water in which Oscars have lived.
This raises questions about the Oscar’s validity for life in a recirculating system conjoined with other tanks.
If both predator and prey are on the same system the prey could live in a state of perpetual terror at the prospect of being eaten by an unseen yet apparent foe!
Never use water from an Oscar tank to top up or replace that from a tank of smaller fish that might constitute an Oscar’s meal in the wild!
Pick on a puny!
Oscars like to make visual evaluations of rivals before picking a fight and are less likely to mix it up with a fish over 75% of their own size, or larger than the size they are.
Signs of a beaten, submissive fish — aside from avoidance — involve a change of colour, becoming much darker, with irregular white or light bars.
In such a situation, separating antagonist from subordinate is the only real solution to avoiding long-term damage.
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