Astronotus ocellatus can charm - both with their looks and behavior. It is easy to want them suddenly and irresistibly, but it is also essential to remember that we buy a living, and long (even 25 years) living animal. It is important to provide them with proper living conditions to avoid hard moments in our existence together.
While planning the aquarium for Oscars we have to keep some of their features in mind: they are big (up to 35cm), they are dirty (according to their size, metabolism and architectonics aspirations) and fairly aggressive, especially in too small tanks, so their apartment MUST be big enough for their demanding. Trying to minimize their aquarium size usually finishes with disastrous quarrels among the fish or trouble with keeping water clear, being polluted by the fish too quickly. The capacities given here must be though treated as minimal. Those who start their adventure with Oscars are kindly asked not to lower them. Only those breeders who know a given specimen can risk such a thing. Exception to the rule is growing very young fish or a quarantine, but the immediate growth of the fish demands quick change of the tank. All the sayings that "A fish fits its size to the tank which it lives in" are invalid in case of Oscars. None of the breeders would like to have small, deformed fish. It is possible to grow them to full size in very small tanks, but such fish are exhausted with the lack of swimming space.
While setting the tank for Oscars we should pay attention to the largest possible area of the tank bottom, especially the lenght of the aquarium, if we want to keep more than one mating pair of the fish. The width of the tank should enable the fish turning without effort-it should not be less than 60cm. The height is not a priority, but the fish of this size should have at least 50cm of glass height with the minimal layer of gravel.
We do not recommend to keep one fish. Lonely Oscars do not show all of their beauty and often have abnormal behaviors like attacking the decorations in the tank.
The mating pair of fish needs 450 litres of water (optimal sizes are 150cm of length, 60cm of width, 50cm of heigth). Two calm pairs should be kept in at least 600litres tank, possibly 200cm long (200x60x50cm). Any extra centimeters of bottom length and additional decorations can help the fish to state the borders of their territories and increase the chances of a peaceful co-existence. Larger groups of fish need relatively larger tanks, as long as possible. If the group is kept together from the early days of their existence, after maturing the fish can perform not only like a typical territorial cichlids, but can show many more interresting behaviours.
Water - Astronotus ocellatus is not as problematic fish if we consider water parameters. Its natural environment is quite soft water (CH up to 4, GH up to 10, ph 6,5-7,5) but it tolerates standard Polish tap water, harder and more alkaline. Even if the fish are resistant, we should monitor the nitrogenous metabolism by-products. Ammonia and nitrites level MUST be 0 (it proves the proper filtration to a given number of fish), nitrates should not be more than 50mg/l. It seems to be a problem, because the norms for drinking water allow such a concentration of nitrates. If we do not follow the rules, our fish will eventually get disease and so common for Oscars HITH among them.
Filtration plays the leading part in keeping the proper water parameters. Apart from neutralizing ammonia and nitrites to least toxic nitrates the filter has to cope with detritus coming ex. from the fish digging in the soil. If we base the filtration on the typical methods the best option seems to be installing 2 external filters, one for mechanical and the other for biological filtration. A good idea is to line them up (mechanical first) - it cancels the necessity of losing a part of biological filter's capacity for a mechanical pre filtration. Apart from the filter media we can put sponges on the filter inlets, but only if we clean them regularly. In this link we should clean the sponges on the mechanical filter inlet every 2-3 days, sometimes even more often, then the sponges from inside of the filter. They should vary from those with big holes to the dense ones to stop even the smallest particles of the detritus and be cleaned every 2 weeks up to one month. If the water flowing through the biological filter filled with ceramics (caramzite for example, but only if it was not treated with fungus killing chemicals and does not change the parameters of water drastically) if free from mechanical pollutants, it should be cleaned very seldom. The working capacity of the filters should be treated as three times lower than the producent suggests, so we buy the filters as if we had tanks three times bigger than we really have.
Oscars is a huge fish (weight up to 1,5kg!) and apart from the urine it produces a lot of excrements and brings up a lot of detritus while digging.
Those people who have a sump filter should keep in mind those quantities of detritus and provide effective pre filtration - a big part of sump should be filled with sponge as a mechanical filter, preventing the biological part from stocking. It seems logical (in addition to the sponges in sump) to add an external filter working only as a mechanical filter.
An interesting idea in Oscars' case is a hydroponic filter - and the best place for it is the first one in the line of filters. In that position it would stop some of the dirt and detritus and utilize them by the plants. They would also take care of the all forms of nitrogen, including nitrates that cannot be utilized by a typical oxygen filtration.
No matter what filter we have, the water changes are essential. There are 2 possibilities - either we do it manually (a given quantity of water from time to time, usually 10-30% every week) or automatically (a constant water change system). A water change is essential to remove nitrates and other substances which cannot be utilized in the tank. The bigger the tank (and the smaller number of fish), the lesser the threat of a collapse of an ecosystem in it, even if we are negligent.
The next parameter keeping our fish healthy is temperature. Oscars tolerate a vast range of temperatures - full-grown fish can live in 18-30 degrees centigrade. They can thrive in lower or higher temperatures but we should not overuse it, especially if the fish are very young or were kept in "comfort". The temperature range means that for the most part of the year in Poland the tanks do not need heating. The heater with the thermostat enables us to fix the minimal temperature (in the author's tank it is 21 degrees) and you can forget about the heating. The heater creates only one problem - the adult lonely Oscar tends to make up different activities. Whatever can be moved or swung will be moved or swung for sure. Broken heater and thermometrs are very common though. So if they must be inside of the tank (sump owners do not have to worry about that) they must be secured and kept away from the animals' reach.
The oxygen amount in water drops with the rise of temperature - it can be seen by the way fish breaths in low temperatures (lazily, almost without moving the gill-covers) in comparison with high temperatures (faster and more hectic). The water for Oscars should be richly oxygenated - if it cannot be done by the water movement caused by the filtration we must use some extra devices (ex. an air pump).
The light does not have to be bright if we do not grow water plants. It even should be dim, with the areas of semi-darkness - it adds some mystery and fish feel definitely better than in the bright tank. In my opinion spot lights are the best if we talk about the variety of effects we can create (lightbulbs, halogens), it does not mean though that we cannot use fluorescent tubes. If we often take photo of our fish we should consider 2 sets of lights in the tank cover - everyday light and the other just for photo shooting.
If we talk about the light intensity the bright light is usually installed when we try to grow water plants together with the Oscars. Apart from the fact that plants need some more than light to grow properly, keeping them in the Oskar tank is very hard to achieve. It is caused mainly by the animals' need to remodel the space around them. According to them the plants always grow in the wrong places :)
The result is the sight of the plants flowing throughout the tank or those which were secured from being ripped out - being destroyed. Oscars are definitely carnivorous so they can shred even the plants with hard leaves - because they felt like doing that. That is why only the "mobile" plants can survive with Oscars, those which can be moved from place to place by the fish. I would not recommend moss ( even if they can be moved) because they collect detritus. So we are left only with floating plants - my favourites are Ceratopteris ferns - they grow fast, need only a little light, the damage done by the fish is slightly visible and they absorb nitrates effectively.
If we treat the plants as an element of decoration, we should remember about its other elements. It should be based on stones and roots, placed and firmly fixed to the bottom according to the owner's taste. Especially stones cannot be movable to avoid the risk of breaking the bottom if the fish dig or move them. So if we use stones, they should be big and placed horizontally and not as a vertical constructions. Oscars do not need caves, they use dark places to hide. And they rarely want to hide. The stones and roots should not have sharp edges. While running away from the threat (usually imaginary) the fish can hurt themselves. That tendency rises with the number of the fish in the tank and their behavior in the group. The fish tend to copy their behavior, so the fish which started the fuss (with ex. biting another from behind) usually finishes with swimming in panic with the rest of the group.
Another social behavior of Oscars is digging in the ground. It appears during mating and when the fish are bored. They need a proper soil to dig in. I recommend fine sand. It does not hurt the fish, it is easy to dig into and it does not let the dirt stay In it. The detritus stays on the surface of the sand and can be easily collected. Of course the aquarium gravel can also be used.
If we fulfill all the requirements mentioned above , add good food and a little attention, we will provide all the fish need to grow, develop and live happily. We will end up with healthy, beautiful and big specimen - and so I wish to all of you.
The author: Rafał Wasiak
(based on his own experience)
Translated by: Agata Suchocka-Wachowska (apricort)