I had two full grown tiger oscars that I took into my possession last November. They had been together in the same tank for at least 2 years, maybe longer. From what I observed during the time that I've had them, they've gotten along really well. They eat, sleep, and swim together with no signs of aggression of fighting that I've noticed (and I watch the fish a lot). Two weeks ago I woke up to find that one of my oscars was beaten very badly overnight and the abusive oscar was still biting at him. They are the only 2 in the tank (100gal). I separated the oscars, but he died a couple hours later. My first question is what would provoke the one oscar to suddenly start beating on the other so severly? No changes had been made in the tank, no addition of new fish.

Since the death the one oscar, the other seems to be bored. He doesn't eat as much or as vigorously as he used to, and he doesn't swim around so much. After checking a website on fish compatability, I tried pairing him with my jack dempsey. Even though the jack dempsey is submissive in the other tank, he quickly took over the oscar tank and became very aggressive, so I immediately put him back into his previous tank. I have a jaguar cichlid, but she's too aggressive to put in with any other fish. I also have 3 dempsey/jaguar hybrids, but I haven't tried to pair any of them up with the oscar yet. I'm confused because the oscar seems to want company, yet he killed his previous long-time roommate for reasons unknown. Should I try putting one of the hybrids in his tank to see if that works out, or should I leave him in the tank alone?

Last question is, I've noticed over the past few days that I have what appears to be brown algae starting to grow in the oscar tank. I've never had this in any of my fish tanks. What causes brown algae to grow and how do I get rid of it?

I apologize for giving you so much to read, but I thank you in advance for any advice.

Oh, and if giving my oscar another tankmate is a good idea what would be the best types of fish I should try to pair him with?

Heya tankmonster!
I read a book yesterday on breeding convicts.
It stated that if a male and female don't get excited at the same time before doing the lovydovy stuff, the male may feel rejected and could kill the female instantly.
Could be something different in your case, but i just had to think of that.

On lonely Oscars:
Mine had a tankmate, a green sunfish and the 2 of them were pretty close.
When the sunfish died ( ) my O started moping and sulking and didn't seem to be his normal self for a long time.
Eventually i decided to get him a tankmate, a big female Vieja.
They didn't like eachother and my O stayed at one side of the tank for 95% of the time. In the end the Vieja beat up my O pretty badly, even had him knock himself out once.
Then i rehomed the Vieja and within a day my O took over the whole tank as his territory.Saw an incedible change in behaviour.
He made it pretty clear that he wants NO other fish in the tank anymore, than the ones he has now. Adding tankmates is allways a guess, and will certainly not allways bring out the best in your fish.

Brown algae can be caused by overfeeding or to many lighthours.
but you say there was no change, so that's a bit strange.

Gladly returning a lot to read to you!

tankmonster wrote:


Silver Dollars are my favorite.
When in a group they look nice, they will not do harm to your O and if he wants he can chase them around as much as he likes without damage.
My O's favorite game is chase the Dollars!

At least, I think it's brown algae. It's brownish colored and looks like hair growing off of some of the plants and decorations. Overfeeding may be possible, but I'm not sure because the planaria I used to have a problem with is gone since I've been doing regular water changes and being careful not to overfeed. The tank lights I leave on for about 5 hours an evening because I read by fish-light at night.

As for the oscar feeling rejected, that seems possible. I did notice a few weeks ago that the oscar that's still living was clearing gravel from one side and underneath one of the decorations in the tank. I was told that it could be a sign of preparing to lay eggs, though no eggs were laid.

Then my assumption may be right, but I'm sure some of the others will share their thoughts on the matter.
I guess you'll need some elbow grease to clean the algae.
My fake plants get totally covered in algae, and i have to work hard to get that off.
i see no definate reason why it started growing with no changes in the tank.

Hard to comment on the O that died, as you will know could be one of loads of different reason, re tank mates, if you do decide for another tank mate, you probably need to change the tank around a lot, especially if the O is "the boss"
add some plants? give them something else to concentrate on, maybe have a tank partition ready for any bust ups,

Maybe not a good idea to add another Oscar, also if adding anything partial water changes as well as above, anything to keep them occupied, i can imagine your tank must look a bit empty at the moment, hope you can find a solution.

I've read that Oscars get aggressive towards one another when they mature and get quite agressive while spawning. But I've never heard one killing its mate although I'm sure it happens. As for tankmates I like Noddy like silver dollars, and my O also plays chase the Dollars. Seems like a popular game.

sorry about your loss:(

sorry to hear baout yr oscar buddy but as far as tank mm8s go i like green/gold severums or as other said silver dollars...maybe some clown loaches? (all large as u will know)

Its difficult to say why your oscars decided to get aggressive. However we need to know a few things to help us. Can you test for ammonia, nitrite, nitrate and Ph. If you get them tested in a LFs can you post the actual results, even better get yourself a test kit and test yourself. How often do you change water and how much?,Do you clean the media in the filter in tank or tap water

Oscars dont need tankmates, in fact some are very tank proud and see the tank as their own and wont tolerate any fish inthe tank