Why is My Oscar Fish Stressed?

Oscars are pretentious fish that require more attention than other species despite being relatively easy to care for. The problem with Oscar fish is that their living requirements sit at a different scale compared to other tank fish like guppies or Bettas.

For instance, one Oscar alone requires at least 55 gallons of water to thrive and remain healthy and stable over the years. They also need some entertainment to remain mentally active since Oscars are highly intelligent and resourceful. And they don’t do well with poor water parameters.

These aspects can make Oscars more difficult to look after in the long run. Fortunately, they’re also more rewarding, given that your typical Oscar will live between 10 to 20 years with proper care. Sometimes even longer than that.

So, what are the main factors that could stress out your Oscar, and what can you do to solve the problem?

Reasons Why Your Oscar is Stressed

Oscars are generally adaptable fish that can cope with a variety of unfavorable situations. Sometimes, however, they can experience stress which will influence their behavior and even their health. The most noticeable factors that will stress your Oscars include:

– A Small Tank

Oscars are big fish by aquarium standards with an impressive growth rate for any fish standards. Oscars can grow up to 1 inch per month until they reach adulthood, at which point they might measure 10 to 14 inches in length.

This is a big, bulky, and strong fish that needs a lot of water volume to feel comfortable. All specialists recommend at least 55 gallons of water for one Oscar, which leaves little room for other fish in the same environment. The problem is that not all fish keepers abide by this rule. Some will even have 2 or 3 Oscars in a tank that should only accommodate 1 fish.

This can lead to overcrowding which will stress your fish and make them more aggressive than normal. You might see your Oscar(s) displaying aggressive behavior towards each other, show low appetite, or even appear depress with low levels of energy.

Increasing the tank’s size is the best way to prevent these problems.

– Low Water Temperature

Oscars are cold-blooded creatures, which means they cannot regulate their internal temperature. Instead, they rely on the environmental temperature to self-regulate their own body temperature.

In nature, Oscars thrive in warmer waters with temperatures varying between 77 to 82 °F. Their minimum and median temperature is higher than that of guppies, while the maximum temperature stays the same. This isn’t to say that Oscars can’t withstand lower temperatures.

The absolute lowest temperature that Oscars can survive in is 55 °F. However, they can’t do it for too long. Oscars subjected to this temperature will die within several days to weeks. If the temperatures aren’t low enough to kill your Oscar, they will stress your fish and weaken its immune system along the way.

This will render Oscar vulnerable to bacterial infections and parasites over time.

– Bad Water Conditions

The problem to remember here is that Oscars are big fish that will produce a lot of waste. Keeping them in a smaller tank will cause them to experience an unstable environment that can become foul fast. Dirty water and an unclean tank will increase the ammonia levels in Oscar’s environment, stressing the fish and even killing it if the situation remains unaddressed.

Oscars require regular water changes to remain healthy and active over the years. Otherwise, ammonia buildup will stress them, weaken their immune system, and poison them if the situation becomes severe enough.

To prevent this situation, change around 10 to 20% of the Oscars’ tank water weekly. You may need to perform more frequent tank water changes if the tank is too small, you have too many fish, or Oscars produce more waste than normal.

Be wary, Oscars tend to produce more waste than other fish species, which can be a problem for smaller tanks unfit for them.

– Bully Tank Mates

Oscars are rather aggressive fish that don’t cope too well with other tank mates, no matter their species. This can lead to aggressive behavior and a more tensed tank dynamics, especially if Oscars aren’t the only aggressive species in their environment.

Oscars don’t do well with bullying or sharing their space with fish species that they’re not compatible with. Not only will bullying stress the Oscars, but it can also lead to injuries, rendering your fish vulnerable to infections.

– Disease or Parasites

There are 5 common diseases that Oscars face more often than others. These include:

  1. HITH Syndrome (Hole In The Head) – This is a disease with varying causes, ranging from poor tank conditions to nutrient imbalances to inadequate nutrition and even the Hexamita parasite. The main symptoms include holes and sores on the head, lack of appetite, and a mucous substance trailing from the holes.
  2. Ich – A common disease in most aquarium fish caused by a protozoan parasite. The symptoms include white spots covering the fish’s body, static swimming, frenetic gill movements, or rubbing their bodies on various tank decorations.
  3. Fin Rot – Another common fish disease attributed to several bacteria. The Oscar is more prone to contracting this disorder when stressed by environmental factors like poor water oxygenation, overcrowding, and fish waste accumulation.
  4. Popeye disease – This disease is the direct result of prolonged poor water conditions, and it’s easy to diagnose. The Oscar will display bulging and cloudy eyes as a result of bacterial infections, fluid buildup behind the eyes, and cornea damage. The condition is notoriously difficult to combat, involving several treatments like quarantine, antibiotics, impeccable water conditions, and aquarium salt.
  5. Dropsy – Dropsy causes a bloated appearance and sometimes triggers other conditions like Popeye’s Disease. The problem is that this condition is very difficult to treat as it often turns lethal.

All these conditions will influence your Oscar’s behavior, causing it to appear drained of energy, reclusive, stressed, or even aggressive.

– Lack of Entertainment

This is an area that few people expect. The truth is that Oscars are highly intelligent fish, constantly on the lookout for entertainment and keep themselves physically and mentally active. It’s for a reason that they’re called water dogs and you should pretty much treat them as such.

Throw a ping-pong ball into their tank and let them chase it for a while, or place their tank next to a mirror to make them think they have company. You should also decorate Oscars’ tank with plants, wood, caves, and rocks to mimic their natural environment and give them something to explore.

How to Keep Your Oscar Fish Happy?

You should first learn the Oscar’s way of life to understand how to provide it with optimal living conditions. This alone with prevent many of the problems I’ve mentioned throughout this article.

If problems do occur, identify and address them immediately and accordingly. The more you ignore them, the faster your Oscar’s composure will break down, leading to the fish displaying a rugged look with pale colors.

Here are several pointers to consider:

  • Provide Oscars with enough space so they can live comfortably (at least 55 gallons for one fish)
  • Clean their tank regularly and perform 10-20% water changes every 5-7 days
  • Watch out for symptoms of disease, parasites, or bacterial infections to treat them in time
  • Monitor water parameters to prevent any imbalances in the long run
  • Monitor Oscar’s behavior in the presence of other tank mates
  • Provide Oscars with a balanced diet


Oscars are hardy fish but will react negatively to prolonged suboptimal environmental conditions. You need to provide them with a stable habitat to keep them in good health over the years.

In return, the Oscar will reward you with its energetic and exotic presence, hopefully, for decades to come.

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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