Oscars are a special fish. As anyone who has kept Oscars can tell you, they are a very intelligent fish with distinct “personalities”. The author had one Oscar that would kill anything put in his tank, literally. He killed a Jack Dempsey that was bigger than he was. A Siamese cat accidently fell into this Oscar’s tank. The Oscar attacked the legs of the cat, snapping away completely unafraid (I laughed my butt off!). The author also had a very mellow large adult Oscar that lived for many years side by side with a feeder goldfish that was supposed to be food.
Oscars are decidedly “drama queens”. Often they will lay over on their sides and look like they are dying. Five minutes later they are fine. Move something around their tanks and they will often “act out” in many ways. And it is common for a new Oscar in a new tank to refuse to eat for two or three weeks. This is just an Oscar being an Oscar. Just wait them out. And be careful with your fingers around any large Oscar. They will bit and the bite can draw blood.
A common question about Oscars goes along the lines of “Can I keep four full grown Oscars in a 100 gallon (378 liter) tank?” And there is no good answer. As alluded to above, Oscars are very individualistic. With adequate filtration possibly 90% to 95% of the time four adult Oscars will work in a 100 gallon (378 liter) and everyone will be mellow. But 5% to 10% of the time you will end up with some dead Oscars from aggression. So an exact prediction can’t be made.
Oscars are a slow-moving fish which will do well in much smaller aquariums than thought. Stocking Oscars is just OPINION, nothing more. If one wants to put a couple Oscars in a fifty gallon (189 liter) go ahead and do it. The stocking level will still be about 5 to 10% of the stocking level of many aquaculture operations and it is not inhumane, in my opinion. But then I’m not a member of PETA USA (LICG NL).
The aquarium below has a bunch of very healthy large Oscars in crystal clear water. IN MY OPINION, there is nothing wrong with this tank and its stocking.
Oscars Need a lot of Biofiltration
Oscars have some special needs. Oscars are a very heavy fish that eats a lot of food and need a HUGE volume of EFFECTIVE filter media. If they don’t get this over-filtration they are very prone to hole in the head disease. This appears to be due to the fact that Oscars in the trade are descended from fish that were taken from blackwater rivers.
Everyone wants to know What is the minimum aquarium size for an Oscar? This is the wrong question. The right question is What is the filter volume and filter media that an Oscar needs?
Note that this volume is not needed for ammonia oxidation. It is needed to provide very bacteria free water. Blackwater has very little in the way of bacteria in it. So blackwater fish like Oscars need very clean, crystal clear, bacteria free water. This is examined better in this link:
Many people house Oscars in aquariums with far too little biofiltration with the wrong type of filter media. This results in Oscars getting hole-in-the-head disease very often. The causes of hole in the head are many but topping the list is inadequate filtration. For an analysis of hole in the head go to this link:
Note that Oscars frequently get hole-in-the-head after their diet is changed. And whatever the new food is get blamed for the hole-in-the-head (HITH). A common refrain is “I started feeding krill and my Oscar got HITH, so obviously krill causes HITH“. It was not the krill that caused the hole in the head. It was the switching of the food.
SOME Oscars don’t like to have their food switched. They like the same food day in and day out. When you switch their food they often just grind up the new food and spit it out uneaten. You can almost see them mouthing the words “you changed my FOOD? You $%#@!**&%#, spit pheweehhhe, spit”. This uneaten food feeds a bacteria explosion and the bacterial explosion creates hole in the head, often quite rapidly. Of course other Oscars eat anything put in their tank.
Filtration for Oscars
Hobbyists fail to appreciate that required biofiltration is not dependent on the length of a fish but rather dependent on volume and weight. And Oscars have a lot of volume and a lot of weight for every inch in length.
This dependency is what is called a “cubed relationship”. This “cubed” relationship means a 12-inch (30 cm) fish like an Oscar is eight times heavier than a 6-inch (15 cm) fish like a zebra mbuna. One twelve-inch Oscar will run about three fourths of a pound to one pound (340 to 450 gram)and needs at least half a cubic feet (14 liter) of foam, pot scrubbers or K1 biofiltration to be very healthy (an FX6 canister will do this).
Note that all filter media are not the same with respect to biofiltration capability, by up to a factor of eight. One twelve-inch (30 cm) Oscar would need one half to one cubic feet (14 to 28 liter) of a good media like 20 ppi foam, plastic pot scrubbers or static K1 media in order to be healthy (that is one to two Sunsun 303s canisters).
One twelve-inch (30 cm) Oscar would need two to four cubic feet (56 to 113 liter) of ceramic rings, Matrix, bioballs or lava rock to be very healthy (that’s four to eight Sunsun 303s canisters). The efficiency of various filter media was tested and ceramic rings, lava rock, Matrix and Biohome did very poorly. The following is the link to the test:
Obviously, one should never keep Oscars with filtration by something as small as a hang-on-back filter or a sponge filter. The Oscar will get hole-in-the-head if that is done.
Note we did not mention water turn-over or flow. These are relatively unimportant in filter sizing. It is the volume and type of filter media that is important. This is analyzed further in this link:
One and a half pound (680 gram) of fish (two twelve-inch (30 cm) Oscars) is the equivalent of roughly thirty six 5-inch (12.7 cm) mbuna in a 70 gallon (265 liter) aquarium and is heavy stocking. A 70 gallon aquarium size wise is large enough for two adult Oscars IF and ONLY IF it has a HUGE amount of effective biofiltration. I like to say think one FX6 canister filter for every adult Oscar.
Because Oscars are a blackwater fish from very bacteria free water, the biggest single factor with the well being of Oscars is the bacteria count in the water. Low bacteria count in the water gives crystal clear water. Large amounts of biomedia in the filter give bacteria free water. So Oscars need crystal clear water and large filters. It is all about bacteria free, crystal clear, over filtered water with Oscars.
It is important to note that bacteria count is logarithmic. “Dirty water” might be ten million bacteria per milliliter while “clean water” might be 10,000 bacteria per milliliter. Going from ten million to five million with a fifty percent water change is NOT going to do a lot for any fish. Contrary to popular mythology, water changes are NOT a panacea for any fish.
It must be emphasized that Oscars are ONLY touchy with regards to the bacteria count. They do fine in a wide range of water hardness, pH and temperatures. The river my home is on in South Florida has hard alkaline water and literally millions of Oscars in it. They are also nowhere near as sensitive to ammonia, nitrite or nitrate as hobbyists seem to think.
Belief Perseverance Effect
Note that many points made in this article go against conventional wisdom as parroted over and over in social media. So be it. The science is firm.
Many will say the above analysis is incorrect. And they will maintain that belief even when presented with five or more university papers or solid experimental evidence saying it is so. They will rationalize and rationalize some more and reject the scientific evidence. This is the psychological phenomenon called “belief perseverance effect” which EVERYONE does and is simply a fact of life which we can’t and won’t try to overcome.
Many of the Oscar “experts” who read this article have come out on social media and said things along the line of this article is “stupid”. But of course they obviously will have never read the supporting links about the points they have issues with. Good old “ignorance is bliss” personified.
“The good thing about science is that it’s true whether or not you believe in it.”Neil deGrasse Tyson