Need HELP Please Trying to keep two Black Arowanas from Fighting!

Originally Posted by vincent.b

Or add more Aro's but try to get those bout the same size as the largest aro inside the tank.

What is the least number of arowana's.
I was thinking of buying 2 silver arowana. And mix them with my two black arowana's.
But someone told me that silver arowana are more aggressive. So I'm not sure what to do.