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Oscar Cichlids: Astronotus Ocellatus

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Fish Aficionado
Sep 20, 2004
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The Popular Wet Pet
Astronotus ocellatus exploded into the aquarium industry and the demand never lightened. These fish proved to have the traits and character many people sought, and it’s no wonder why. They’re easy to keep and breed, and in turn are easy to find and cheap to buy. They not only get big, but they also grow extraordinarily fast, appealing to the impatient. They have the typical attitude of cichlids yet are not overly aggressive. They’re very intelligent for fish and exceedingly outgoing toward their owners. In fact, their ceaseless appetites give the idea of a begging dog, something fun for the ‘wet pet’ fans, and good for those who can’t seem to stop feeding their fish. Any well cared for Oscar can live for ten years, and they can reach over fifteen, so the Oscar might be considered a long term commitment.

Big fish, big tank
The problem with the popularity of these fish is that the sales of Oscars versus the sales of large size aquariums don’t match up. The maximum size of an Oscar is at least a foot, and males can reach larger lengths then sixteen inches. In the first few months the Oscar will grow from an inch to half a foot, and within the first year they typically reach eight to ten inches. At this point the growth slows and it takes the rest of their lives to reach their maximum potential, as they don’t really stop growing until they die. Oscars are strong fish with plenty of meat on them and once we take into account their impressive size and combine this with their voracious appetites, it’s easy to imagine what’s coming out the other end of the fish. These are not clean fish to keep.

The Oscars rapid growth means the idea of buying a small tank and upgrading later is not a good idea. If you don’t have enough space for the adults, strongly consider holding off the purchase until you do.

How big is big?
The minimum common opinion for a single Oscar is in the range of sixty to seventy gallons. A tank with a depth of 18” is a good idea, since most adults will grow longer then the depth of a typical 12” tank, which limits the Oscar to a very two-dimensional world. Instead of going with the 55g four foot tank, perhaps it’s worth looking at the 60g three foot tank, which has the depth we’re looking for. Better still, why not just do it right the first time and go for the 48”x18” tank and get the best of both. Height is not really important (within reason), but remember that water volume is always a good thing.

For a pair of Oscars the 48”x18” works well, and with enough space to keep it a 24” depth would have the added bonus of space and volume, and make for an impressive display. For a group of Oscars we are now looking at over 240 gallons or ponds.

Oscars are simple fish and require a simple set-up. There are a few points worth mentioning in the setup of the tank – these basics are enough to keep a happy and healthy Oscar for a long time to come, though most will go further to make a more impressive aquarium.

Filtration: We need enough filtration to keep up with the mess, so double or even triple the amount that would be used on a typical tropical setup. Any type of power filter will work; the important thing is to have enough.

Heater: Oscars should have the water warmed to about 78-80 degrees, but they are notorious for attacking and breaking the heater, so a heater guard is a worthwhile investment.

Thermometer: Anything but glass.

Cover: Oscars can and will get overenthusiastic and jump, so a heavy or tight fitted cover is a must.

This is all it takes to keep a healthy and happy Oscar. Juveniles should have a place to hide, but before long they won’t need it anymore. You can go with sand, gravel or bare glass. Choose whatever lighting you like best, and any safe décor you fancy, as long as there’s ample space for the fish to move. Oscars will move and tear up plants, and fake ones, if any, should be used.

A pair, not just two
It is important to establish for those considering more then one Oscar that a pair is not merely two fish. A pair is a male and a female that have formed a mating bond. If we randomly choose two Oscars and place them together we could end up with two males, which will inevitably come to blows as they grow, or two females, which will often result in the same. Even if we get a male and a female they may not make the connection, or one might not survive the aggression it can take to establish it.

The best way to safely and surely gain a pair is to start off with at least four fish. As they grow, changes in their behaviour will be noticed. Their juvenile grouping instincts will diminish and their territorial instincts will emerge (Oscars generally keep some of their grouping instinct, given enough space). By observing the tank it becomes easy to tell when two fish have established a bond, at which point the remaining fish are best removed. The fish are still young and anything could happen, but the chances are now on our side that these two fish will continue their friendship through adulthood.

Oscar and company
There is no doubt that Oscars are safest kept in species tanks, but because they are not so much aggressive as simply big and hungry, there are some choices for companions if the tank is big enough. An Oscar raised alone will often fight to keep the space and introducing new fish could become a challenge, so it helps to introduce any tank mates while the Oscar is still young. Keep in mind that a pair of Oscars will be much more territorial then a single one, and in these cases a very large tank will be needed to keep more then just the pair.

The two factors that make a fish incompatible are these:

Threat: Oscars, like most cichlids, take their territory very seriously, so any tank mates must not threaten their established space. Territorial fish like other cichlids will fight for the same space and Oscars are not nearly as tough as they look compared to the average cichlid. Although mixing cichlids or other territorial creatures might work with a bit of ingenuity, and a healthy dose of luck, it’s certainly not how to bring out the best in our Oscar. Although other large new world cichlids are among the most popular choices for Oscar companions, they are not necessarily the best.

Food: This one is quite simple. If the Oscar can eat it, it will eat it. Any companions for an Oscar must be bigger then its mouth by enough size that they wouldn’t even be considered food.

When choosing what to stock with Oscars also keep in mind how much faster they grow then the average fish, so give any tank mates a jumpstart on size. Just be careful they are not added before the Oscar is old enough to handle itself. Some of the commonly kept companions of Oscars are:

Silver Dollars: The tall, compressed shape and larger size of these fish make them near impossible for an Oscar to eat, and their peaceful, herbivorous nature makes them no threat to the predatory Oscar. The need to keep these shoaling fish in groups is more detrimental then ever, since the Oscar is bound to show some aggression toward them from time to time.

Bala Sharks or Clown Loaches: These bottom feeders are big and tough enough to handle the Oscar, but they both reach lengths close that of the Oscar and need to be kept in groups, so it takes a very large aquarium to take this route.

Tinfoil Barbs: There is no doubt these fish are big and tough enough to handle it, as long as we keep in mind they can also reach over a foot and also need to be kept in a group. Tinfoil barbs are extremely active and easily spooked, and could bother shier Oscars.

Large Plecos: Oscars will usually ignore plecos and they’re a failsafe choice for many. Even if they are noticed once in a while, they grow large and have tough armour to help them out. The one reason to avoid plecos is regarding cleanliness – we already have one or two big and messy fish, and plecos are among the messiest livestock.

Hungry, hungry Oscar
Prepared pellets are a safe source of nutrition for Oscars, and the convenience is hard to beat, but there’s nothing like fresh supplements of which there are plenty readily available (live, frozen, or freeze-dried) that would be readily enjoyed and nutritionally beneficial for the Oscar. Krill, prawn, shrimp, small fish, crickets, all sorts of worms, and crayfish are some of the more common choices.

Oscars love eating and they need lots of food to grow big and healthy. How much and how often we feed our Oscars becomes a matter judgement and common sense. You might want to stuff your Oscar full of as many prawns or fish as he can eat, but then it might be wise to let a day or two pass before the next feeding. Perhaps you enjoy feedings and want spread it out into smaller portions. Many aquarists leave one day per week to skip feeding altogether with the intention of letting the fish clear its system. This theory makes sense given that in nature Oscars can go many days finding little or nothing to eat, and while there’s no real proof that this practice really helps, it certainly doesn’t hurt.

There is much debate on the risks and nutrition of using feeder fish. It has been found that Oscars in the wild do eat small fish as a part of their diet, and feeders can be a nutritional addition to their diet in the aquarium as well. The problem with feeder fish lies in how we acquire them. The common goldfish or guppies traditionally sold at fish shops are undernourished, making them less nutritional for the Oscar and more susceptible to disease. To make feeders worthwhile they should be quarantined first in order to fatten them up and ensure they have not brought disease from the store.

An Oscars Ail
Oscars are extremely hardy fish and once placed in an appropriate environment are very easy to keep healthy. The two most common health problems with Oscars are Heximita and constipation.

Heximita is a parasite that can lead to a rotting of the flesh on the head and lateral lines of some types of fish, hence the common name to the problem, ‘hole in the head disease’. Deep pits form in the flesh, usually starting in the head area. A neglected condition would increasingly strip the fish of flesh until if finally died. Oscars are especially prone to this problem.

Heximita is found to be carried to some level in the digestive tracts many fish, but a healthy and unstressed specimen has an immune system able to keep it at a safe level where there are no physical symptoms. There are different ideas of what can cause it and there are times when no apparent cause can be found, but the most popular reasons are substandard or undernourished water and a poor or unvaried diet. Other things that might bring it on are stress from an unsuitable environment or inferior genes. Although it is still debated, there is a theory that carbon in the filter can lead to this disease. This makes sense on one level – carbon removes bad elements from the water, but at the same time it can’t pick and choose, and the result is the removal of good elements as well. The safe bet is to keep carbon out altogether.

To tackle the problem, raise the temperature of the water to 82 degrees and add a small amount of salt, about one tsp per 5 gallons of water. Keep the water pristine with plenty of water changes and make sure the diet and environment is appropriate for the fish. The problem should neutralize at this point and over time the condition should improve, but advanced cases may never fully heal. If all of the above steps fail, then it’s time to consider a medication like Metronidazole.

Constipation may occur if the Oscar is overfed with rich foods like meat from mammals or poultry. Pellets are a great source of nutrition, but feeding too many at once can also constipate them because they’re so dense, containing only 10% moisture where natural foods are upwards of 90%. Feed pellets in small portions and avoid foods alien to an Oscar’s natural diet.

A sure sign of constipation is that they are no longer emptying their bowels. The fish tend to continue eating even though it isn’t coming out the other end and eventually could eat itself to death. If there are signs of constipation stop feeding the fish all food and begin a diet solely of peas. Give him a few days to warm up to the peas if he doesn’t take to them at first, they’ll clear him right up. If he won’t take the peas, or the problem has gone too far, an Epsom salt bath would be worth a try, as this is a very efficient laxative. Mix 75g or 2.5oz of Epsom salt per litre, or quarter gallon, of water from the aquarium into a container and bathe the fish for ten to fifteen. Do this daily until the fishes system is cleared.
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