Piranha and Oscar

Question
QUESTION: Hi you said you know about tropical fish so im hoping you know about piranhas and Oscars :)

Ok basically i recently upgraded from a 5 gallon tank to a 20 gallon tank. I put my piranha that i had in my 5 gallon into the 20, and added a new Oscar that i bought at the same time (so neither had a specific territory). They are both about 5 inches long and both are aggressive fish. I have had them both in the tank now for about 2 weeks and i used the rocks and filter from my other tank so the New tank syndrome cycle has finished. When i first added them they both hung out in the plants alot and never came out to eat. Now they are eating fine and stuff, so anyway, my question is just that when they get older and grow bigger, will they still get along? Right now they get into little tussles every now and then i see them nibble at each other till one swims to the other side. I know they both are aggressive so i don't know if later they will eventually try to eat eachother or something. Oh and another question is that i have tested my Nitrite/Nitrate levels and my Nitrite is at 0ppm and my Nitrate is at approximately 80ppm. Is this good? Or will it fad away eventually?

ANSWER: Hi Bill,
Keeping those 2 fish in a 20  gallon is like keeping a child in a cardboard box if you know what I mean. It's too small.

80ppm is way too high, and is most likely because of the large fish in the 20 gallon.

It is hard to say if they will be able to live together when they are older; oscars have very different personalities so I can't tell. If they are fighting at this age, it may be territorial or just because they don't get along.

-Will

---------- FOLLOW-UP ----------

QUESTION: Oh i see what you mean. I recently got a job so i am saving up for at least a 55 gallon tank. As for my Piranha, one of my friends decided to take it so now i just have an Oscar in a 20 gallon, soon to be 55. I read something online about teaching your oscar tricks, is this true? do they eventually recognize their owner?.. I just read online about hand feeding your oscar is a good beginner trick?! Is it safe for me to be sticking my hand into the tank?
Not that i am scared that my oscar will bite me, its more of the water being contaminated from the germs/oils on my hand. I think it would be pretty cool to hand feed one!

Answer
Hi Bill.
The 55 gallon will work; just keep it really clean.

I've been able to teach oscars and other fish the following tricks:
Fetch,
Handfeed,
Soccer,
Slolom,
Basketball,
Hoops and much more. I can teach you them if you want. Oscars are some of the smartest fish so it will be easy.

-Will