Are Oscars Aggressive To Other Fish? (Answered and Explained)

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Oscars have a reputation for being aggressive, but is that reputation deserved? Like any fish, in fact, any animal, each individual oscar is different. One fish may just be a killer, determined to batter any fish that even looks at him, the next may decide he is happy with his lot in life, never bothering a soul.

I have been keeping Oscars for the best part of 30 years and over that time I have had some Oscars which are outright killers and others that have never shown any aggression at all. It really does vary fish-to-fish.

Oscars are certainly big, powerful fish that are capable of showing aggression, but they are also surprisingly gentle when they need to be. Oscars make fantastic parents for instance and will protect their young at any cost. The only way to know if your Oscar is aggressive is to really get to know him.

Oscars can be boisterous. You may see them pushing each other, chasing other fish, or ‘lip locking’ with another oscar. This is all part of their natural behavior.

How To Reduce Oscar Aggression

You can reduce the chances of your Oscar being aggressive by following some of the suggestions below;

  • Choose suitable tank mates
  • Choose the right size aquarium
  • Feed your Oscars well

Choosing Suitable tank mates for your oscar

Choosing suitable tank mates for your oscar is important (we have a great article on oscar tankmates). If the other fish are too small, your oscar will, in all likelihood, try to eat them.

The most important rule for ANY breed of fish is, ‘if another fish can fit in its mouth, one day, it will end up there!’. Oscars are not a fish eater by design, but they won’t pass up an opportunity for an easy meal

If the other fish are too big, they may bully your Oscar, leaving him no choice but to be aggressive to defend himself. The problem with choosing suitable tank mates is that a healthy oscar grows so quickly.

You buy him as a 2” cutie from the pet store, and within 12 months he is a 10-12” monster. We tend to keep our Oscars either with other Oscars or with fish that have no interest in getting involved in the Oscars business (such as a large Plecostomus).

Choosing the right Tank size for your oscar

Choosing the right size tank for your oscar can also play a part in whether or not your fish is aggressive. Cram 5 adults in a motel room for a couple of weeks and watch them turn on one another. The same is true if you keep 3 Oscars in a 40-gallon tank. Oscars need their space (I recently wrote an article titled What Size Aquarium Does My Oscar Need? which may help).

In the wild, they have their own territory which they will defend vigorously. They don’t want to be on top of one another. I won’t usually keep 1 oscar in anything less than a 55-gallon or better a 75-gallon. For more than 1 oscar, I like to see an additional 25 gallons per fish. More is always better. No Oscar ever complained he had too much space.

Feeding your Oscar well

A well-fed oscar is far less likely to be aggressive than a hungry one. Who can be bothered to go and start a fight after they have just finished their Thanksgiving lunch? Keeping your Oscars well-fed, especially if you give them a varied diet, will keep their aggression levels down.

I don’t normally use feeder fish as an oscar can hurt himself chasing down his prey, but I will give my oscar’s frozen and thawed whitebait, shrimp, or mussel meat.

I have a great article on 10 unusual foods your oscar will actually enjoy eating.

Why Are My Oscars Biting Each Other’s Mouths?

Oscars are well known for ‘lip locking’ with other Oscars. In juvenile fish, this is just harmless sparing. They are testing each other’s strengths. They won’t do one another any harm. With adult Oscars, lip-locking may be a sign you have a pair who are preparing to breed.

Locking their mouths together, chasing each other around the tank, or being rough with one another may be a sign the two fish have decided to pair up. Providing one fish doesn’t have the other constantly pinned to the floor or curled up in a corner, you don’t need to intervene.

What To Do If My Oscar IS Aggressive?

If you have decided your oscar just has a really short fuse, there are a number of things you can do.

Firstly, can you move him to a tank on his own? If he lives alone, there will be no one to be aggressive towards (although I have had an oscar that would attack the glass tank sides, presumably because he could see his own reflection). If a separate tank is not an option, can you change your current tank for a larger one? More space may give your oscar the opportunity to have his own territory which the other fish can stay out of.

Alternatively, try adding additional rocks, decorations, or fake plants (Oscars will destroy real plants in no time at all). Adding more decorations will create a line of sight block.

Normally, if an Oscar can’t see tank mates, he won’t bother them. I have a 155-gallon tank with piles of rocks. The Oscar usually stays on one side, while his tank mate stays on the other. It’s a sharing arrangement that works for them both.

Simply rearranging the existing tank decorations can, on occasions, stop an oscar from being aggressive. When we take all the rocks out of the tank and put them back in different locations, it can ‘reset’ the pecking order. An oscar that was at the top of the tree, can find himself further down the pecking order and put an end to his aggressive behavior.

Another option is to add a dither fish to distract your Oscar. Admittedly, choosing a dither fish for an oscar tank can be tricky. I have had limited success with zebra danios. They are fast enough to keep out the Oscars reach, but I did notice the number dwindling over time. In very large tanks a school of Tinfoil Barbs makes good dithers, but they require open swimming space. Don’t swap one problem for another.

A less than pleasing option is to divide your tank in two with a divider. Egg crate (sometimes known as lighting diffuser) can be used as a barrier that stops the fish but doesn’t impede the flow of water around the tank. Egg crate down the middle of your tank doesn’t look great, and a determined Oscar can break his way through.

The final solution to an aggressive Oscar is to rehome him. It can be hard to re-home an oscar. Not just because they become pets, but also because there are so many looking for new homes.

Finding someone who has a tank large enough to rehome your Oscar, and is willing to take it, can be a challenge. If you do find someone, consider making them a donation to cover the cost of feeding your pet.

In Conclusion

Oscars can be aggressive, but they are not normally out and out killers determined to make trouble wherever they go. Look at your oscars’ situation and see if there is something you can change to make him more placid. You will usually find one or two small changes will have a big impact on your Oscars aggression levels.

About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James

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