How Big Do Oscar Fish Get: Size and Factors

Oscar fish (Astronotus ocellatus) is one of the most popular types of fish belonging in the cichlid family that aquarists keep as pet fish. The Oscar fish is native to South America and to the Amazon river basin, which are warm and have a high water flow. When you first get them, they can be quite small but grow quickly in captivity. In this article, we are going to find out the size that this species can get as well as the factors that should be taken into consideration regarding their physical growth.

Oscar fish, tiger oscar, marble cichlid (Astronotus ocellatus).

How Big Do Oscar Fish Get?

The size that an Oscar fish can grow to is about 11-12 inches as an adult depending on the condition of the habitat. When they are captured from the wild, they are only about 1-2 inches but after a year in captivity, and depending on how well you provide for your fish, they can grow for about a foot. In captivity, Oscar fish can get as big and weighted over two pounds, they don’t exceed one pound.

The basic condition for an Oscar fish’s growth is due to their own physical characteristics as well as their diet. They can either grow quickly or slowly because of these reasons, but mostly, it depends on the aquarium conditions and what you give them. The overall length of your fish’s aquarium also contributes a lot towards how big it can get as well as where they need room to spread out because if your fish stays too cramped, then it won’t have space for development.

What Is the Maximum Size of Oscar Fish?

In average, the maximum size an Oscar fish can get is around 16 inches in length and three to four pounds. But again, this will depend on aquarium conditions such as water temperature which plays a significant part into how big they can become. Knowing this, some could get bigger than what we have just suggested but the maximum weight of a specimen will depend on other factors which can include how many fry they might spawn.

How Long Does It Take for Oscar Fish to Grow Full Size?

The longest an Oscar fish can grow on far is around seven years (a little less than 2 inches a year) so they’re not long lived either but as you could expect, growing size happens slowly and over time which a lot of people don’t like to consider because it’s always easier to try and describe how something looks when it first grows up in the tank that shows results right away, especially if the second or third brood come soon after. On average, these will take around five to seven years until they’re the full size.

Tiger Oscar Astronotus ocellatus

How Do You Get Oscar Fish Big Fast?

Let’s go on a look at the factors that should be considered when it comes to their growth rate in captivity:

Tank size

The suggested tank size for Oscars will typically be around 55 gallons with a depth of 24 inches or more. If your Oscar fish lives in an aquarium smaller than this, then you might want start looking at what can get done especially if they’re not living under ideal conditions before you know the reason why exactly their growth rate is slow.

Diet and Nutrition

Oscar fish are omnivorous which means they can eat almost anything that is readily available in an aquarium but they will not be able to find a lot of what these foods contain if their tank is devoid of any food products like live food, vegetables, meat and other types of meaty items or eggs. You should know very well what your pet Oscar eats because without this information, you cannot maximize the development process as optimum while keeping them healthy and environmentally sound.

Oscar fish have a lot of vitaminlike compounds within their system like fat soluble vitamins A, D and E which are essential to good health so it is very vital for them to consume these products since without getting these foods from the food you wish to feed them, your pet Oscar will not be able to develop at an optimal pace. Oscar fish’s diet should be modified from regular canned food or flakes to other kinds of foods like fresh fruits, vegetables and meaty products that are infused in some kind of oil for better nutrition which requires lots more energy if this is done correctly then the feedings will be very necessary as well. This is a good idea if you want your Oscar to grow big fast rather than slow.


Extremely high or low water temperatures limit how big Oscars reach as well as their long-term reproductive capacity. The ideal temperature for this freshwater fish species would be around 74 degrees Fahrenheit to 81 degrees Fahrenheit (23 degrees Celsius to 27 degrees Celsius).

Water Conditions

Oscars will do best in fairly well-filtered and abundant aqueous live organisms with excellent water quality. They prefer calm waters as to not make them stressed or activate their breathing system which would take away from the effectiveness of feeding. In addition, Oscars need good water circulation and oxygenation. The best tank for your Oscar fish is one that has a very low amount of algae, no rocks or decorations and enough light so that they can be seen clearly by the fish.


Oscar fish are widely thought to like low light. In truth, they aren’t as dependent on it than other fish; they will go long periods without hiding away or is even disturbed by sudden changes of their environment. In addition, they will also appreciate having some strong light on the surface of the water. They need at least two hours of sunlight each day, so it is recommended to have them in a tank that has a window or skylight with direct sunlight coming through.


These fish will thrive and breed in a 50 gallon breeder- they can get to be very messy so keep an eye on their water levels. They don’t tend to stay under these tanks and will tear through anything that comes near them. Perform some maintenance regularly such as adding new gravel and cleaning out the filters. Performance of your tank depends only on whether or not you’re doing it consistently. It may not end up being the best for housing but it’s an excellent choice to include in community tanks.

Oxygen and Hardness

The hardness of the water essentially determines how much oxygen and total alkalinity can be present within a certain volume of water, in other words, at what concentration each cubic centimeter would contain enough dissolved oxygen for a fish with gill filtration as its normal mode of respiration (as opposed to air). Normally, the water hardness should correspond approximately to a level of 100 to 200ppm.

Tank Maintenance

The most common problem in maintaining Oscar tanks is the increase of visible algae. The following are suggested measures to maintain water quality:

  • Use a phosphate remover as soon as you can.
  • Cleaning glass or acrylic surfaces will help with algae growth until it becomes a cycled tank and breeding colonies become too large for regular cleaning effectively, then remove the surface from inside by scratching holes through to lugging out  algae mass.
  • Chemical control and regular monitoring are necessary by condition, even proper filtration systems will nevertheless create non oxygenated (redox dead) waste matter. Many have posted about redox state of O3 in ponds amongst other such parameters so needless to expand on this at this time although I feel it is worth noting and a reason as to why goldfish tanks tend technology resistant features that enable stable pollution bottoms excluding direct contact with the water.