Why is Oscar Fish Not Growing?

Oscars are amazing cichlids that mostly do well in single-species tanks. There are several reasons for that, especially due to the fact that they grow fast, they grow big, and they are territorial. As any other healthy, normal, and respectable cichlid, Oscars are extremely aggressive towards any tank companion, especially another Oscar.

So, Oscars are better kept alone, and they require a stable, clean, and healthy environment to grow. Generally, Oscars will grow around 12 inches in captivity and get slightly larger in the wild, around 14 inches. But how fast and how much should they be growing?

The problem here is that not all Oscars are the same. Some grow faster than others, while others become larger for reasons you can’t always control. When it comes to Oscar maximum size and growth rate, there are several factors that could lead to distorted stances on the subject.

Today, we will assess the main reasons why Oscar’s growth may seem hindered or slowed down compared to the norm.

Reasons Oscar Fish Not Growing

If you’ve got yourself a healthy and handsome juvenile Oscar, assessing its growth rate is the best method of grasping its health. Healthy Oscars display astounding growth rates of up to 1 inch per month, given optimal living conditions. This means they might reach their mature size within the first year of life. Which is amazing knowing that Oscars can live up to 20 years in high-end environmental conditions and with a proper diet.

That being said, the Oscar’s growth rate depends on several factors, including:

  • Genetic – This is probably the first one to go to after eliminating all other potential causes. Some Oscar fish come with subpar genetics influencing not only their growth rate but their maximum size as well. This is one of the downsides of acquiring your Oscar from regular fish shops that cannot guarantee for the fish’s genetic makeup.
  • Diet – Oscars are omnivorous, but they lean more towards a carnivorous diet. It’s not enough to provide them with enough food, it also needs to be diverse. This grants them access to all the nutrients, vitamins, and minerals necessary to sustain their accelerated growth rate. Improper feeding and subpar diets will affect your Oscar’s overall health and growth rate.
  • Bad water conditions – Oscars are pretty adaptable fish in the sense that they can live in a variety of water conditions. But they won’t thrive in subpar habitats with fluctuating temperatures, dirty waters, and unclean tanks. Improper water conditions, including cold waters, will hinder the Oscar’s growth and even lead to extensive health problems due to a faulty immune system.
  • Insufficient space – A mature, fully-grown Oscar requires at least 55 gallons of water. You may even need to consider 75 gallons for a 12-inch Oscar with various plants and tank decorations around. The problem is that many people place the Oscar in a smaller tank while the fish is still small, oblivious to this species’ accelerated growth rate. Next thing you know, the Oscar no longer fits its environment. A small aquarium will hinder the Oscar’s maximum size.
  • Disease and stress – Oscars can face a variety of health issues, many of them related to poor diets and improper water conditions. These include Ich, Popeye disease, dropsy, etc. Aside from being potentially deadly, these conditions may also influence Oscar’s growth rate and overall size.

As you can see, everything pretty much comes down to ensuring optimal living conditions, a clean habitat, and providing the Oscar with a nutritious diet. This is generally enough to keep the fish healthy long-term.

How to Make Your Oscar Grow Faster?

When it comes to boosting your Oscar’s growth rate, I would say you have 3 options at your disposal:

  • Diet and feeding – The Oscar requires proper feeding to achieve its growth potential. The notion of ‘proper feeding’ includes aspects like fresh, varied, and healthy food via a feeding patter that mimics the fish’s natural feeding. Oscars need to eat 2-3 times per day during their juvenile phase and once per day at most as adults. Anything more than that may lead to overfeeding, which will affect your Oscars’ digestive system. When feeding your Oscar, consider a well-balanced diet consisting of shrimp, insects, worms, spirulina, and various homegrown veggies, depending what they like.
  • Tank temperature – Your Oscars require environmental temperatures between 74 and 82 F. Generally speaking, the higher the temperature, the faster the Oscar will grow, especially during its first weeks of life. Keep the tank’s temperature stable in the neighborhood of 78-80 F to boost the Oscar’s growth rate. This will keep the fish more comfortable and energetic, accelerating its metabolism, aiding in digestion, and keeping it healthier long-term.
  • Disease prevention and early treatment – You should always monitor your Oscar’s environment to detect any dangerous fluctuations in its water conditions. If your Oscar displays abnormal behavior like erratic swimming, lack of appetite, weird body spots, or lesions, quarantine the fish and consider immediate adequate treatment. Oscars are pretty resilient and will recover from pretty much any conditions, so long as you ensure proper treatment and detect the disease in time. Treating the condition sooner will also avoid growth issues linked to disease-related stress.

Other than that, provide your Oscar with stable water parameters, which demands a heater and a filter. Ammonia and nitrites should remain at 0; otherwise, your fish will experience severe health issues long-term.

How Big do Oscar Fish Usually Get?

Oscars will live up to 12 inches in captivity, given proper food, housing, and long-term care. When it comes to long-term care, the notion includes aspects like:

  • Preventing overfeeding, which may lead to ammonia and nitrite, boosts
  • Keeping the Oscar’s environment clean as much as possible
  • Performing weekly water changes, depending how large and crowded the tank is
  • Avoiding community scenarios, since Oscars don’t do well in community tanks
  • Provide a varied diet and a strict feeding pattern
  • Monitor the Oscar’s water parameters to avoid any dangerous fluctuations
  • Ensure optimal treatment fast in case of showing signs of illness

Providing the Oscar with regular entertainment, which includes a larger tank, aquatic decorations, a mirror, or ping-pong balls for the fish to play with

Is it Normal that One Oscar Fish is Growing Faster Than the Other?

Yes, that’s normal because Oscars are not robots. You can’t adjust their growth rate electronically and watch them meet those artificial standards to the letter. There are plenty of factors influencing your Oscars’ growth, and the most important one is each other’s company. Oscars can be quite territorial and competitive towards each other.

This could lead to one eating more than the other, asserting territorial dominance, or swimming more and consuming more energy. At the same time, males grow faster and bigger than females, so that’s another factor to weigh in. And finally, genetics also plays a role in determining each fish’s growth size and speed.

Generally, if your Oscars appear healthy and show no signs of illness, don’t worry about it. Make sure they don’t fight each other too often, ensure optimal tank conditions, and feed them properly, and they will thrive. At their own pace, of course, but nothing to lose nights over.


Oscars are gorgeous and highly rewarding cichlids that won’t require too much from you in terms of long-term care. They only require a stable and clean habitat and good food, and they will flourish.

I suggest keeping an eye on your Oscar to monitor its growth rate and overall behavior. If there’s no sign of any health issue, your Oscar is fine.

avatar Noah
I’m Noah, chief editor at VIVO Pets and the proud owner of a playful, energetic husky (Max). I’ve been a volunteer at Rex Animal Rescue for over 2 years. I love learning and writing about different animals that can be kept as pets. read more...

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