I have to find out why my oldest Oscar is chasing all the other fish in the tank. I originally had 6 oscars in this tank but over the last 6 months or so, one of them has been chasing the rest of the fish in the tank and so far, he' killed 3 of them. At this time, it seems as if he's trying to kill off the other two as well, so what can I do to stop him from doing this? When I first got the older one (the bully) there were 3 other oscars in the tank with him. Every day for three days in a row, I would come home and one of the others would be dead. He killed off all of the other fish in the tank with him then and he's acting like he's trying to do the same thing now. I have a lot of money tied up in these fish so I really don't really like him doing this to all the rest of the fish in the tank. Seeing how, he is the one I've had the longest, he seems to not want to have any other fish in there with him now. Which seems rather strange since I've lived in this apartment and the 6 of them have lived in the same tank together all this time. So I'm confused about why this fish is acting this way now.
Another thing I noticed about this oldest fish - when he's (she's?) coming up long side one of other fish in the tank he kind of starts shivering or shaking or whatever you want to call it. Is this a sign of mating or is it a fighter's stance? I just don't understand why all this time I've had these fish they have never once laid any eggs. I wouldn't know that they were even trying to mate since I've never seen it happen before. All I know is this fish is acting "crazier" every day. I just need to know what to do about it if anyone has any suggestions.

can you tell us what size is your tank along with what type of filtration you have on your tank. also your water readings for ammonia nitrites and nitrates would help a gread deal.

Well, first of all, I don't know anything about testing for nitrates or nitrites or whatever it was that was you mentioned. I have a 29 gallon tank with an Emperor 400 filter in the tank. The temperature of the water in the tank is right, but I've had Oscars in this same tank for over 5 years now and with the exception of the other time I told you about in my first message, I've never had any problem with this happening at any other time. I try to keep the filter cartridges as clean as I can and replace the water whenever it gets tinted by the food (TetraCichlid Jumbo sticks) that I feed them. I usually buy live goldfish to feed them once a month but other than that they seem perfectly fine. I just don't have any room in my apartment for a tank any larger than the one I have now. I also have 2 - 10 gallon tanks besides the 29 gallon but I have some baby Oscars in one and a blue African cichlid and 2 blue gouramis in the other one. I may eventually buy another 29 gallon tank but I can't do it until I have the money - which may not be for some time yet. I have a small apartment which means that I don't have any space left for anything else besides what's already here. My landlady recently told me she was going to buy a 100 gallon or larger tank to put in the lobby of this apartment building so I told her she could put my big Oscars in that tank - just so give them more space to live. I just don't know when that will be. The babies I have in the other tank are still pretty small so when I put the bigger Oscars in the apartment building's tank, then I can put the 2 largest baby Oscars in the 29 gallon tank. This older Oscar is chasing the other two fish in the tank. When they finally die, they don't have any scales left on them on either side. I just lost one of my tiger albinos a couple of days ago but when I took him out of the tank, he had no tiger markings left on him at all. I have always heard that the male Oscars did that to the females when they tried to mate with them but how do they mate if the male kills the female like that? I just need to know what I can do to try to get this fish to calm down because he's running the other two fish crazy, it seems. I am just trying to keep the other fish alive, so what do I need to do now, just for the time being?

I'm afraid your situation isn't good. 29 gallons is just nowhere near big enough for an Oscar, let alone any other fish. This could be why your Oscar is killing all the other fish, in such a small environment, they can't get away from each other. Whereas Oscars aren't normally the most aggressive fish, they're not really sociable with other types of fish.

You really have only two options, either upgrade your tank to a much larger one, or alternatively, only keep small community fish that can live together in harmony.

That's probably not what you want to hear, but that's the only advice I can give you. The fact that you've kept Oscars in a 29 gallon tank for the last five years is neither here nor there, it is not big enough for an Oscar. If you carry on keeping your fish like that, you will forever be coming on here asking for advice on how to sort out problems. I have two tanks, a 300 gallon, and a 125 gallon, I don't have any problems whatsoever. That's because the fish are plenty of space.

Like OFL has said, 29 gallon is nowhere near big enough, you need at least 55 gallon for 1 oscar alone, let alone tank mates.
also, its very very inportant to get yourself a water test kit. you have to keep track of your water levels (ammonia, nitrate, nitrite, ph)
the answer to your problem is already there, your oscar is attacking because he dont want any tank mates in a tank that size. oscars grow to an average of 14-16inches in the right size tank, so at this moment in time, i doubt your oscar is happy.
you really do need to either get a larger tank or think about rehoming your oscar.
what other fish do you have in your tank at this present time?
i know this aint what you wanted to hear but we are here to help you.
i hope you do the right thing.

i concur with OFL.

courting would be impossible in that size of tank.
definitely, what you have there is survival for the
fittest.