Well lets just put it this way, I have two tiger oscars (one black, one albino) both roughly 3.5 inches and a monochromis that is tiny. I went and bought a 5 inch Australian arowana and a 8 inch clown knife fish and put them all in the same tank.
Now before purchasing my arowana and knife fish i asked various 'professional' fish keepers if the arowana would be fine with the oscars and monochromis and explained that my oscars could be pretty boystrous when they wanted to. The answer i got was yes the arowana was very aggressive if provoked and would do just fine in the tank.
So anyway put my new fish in the tank, went to work, came home, thought i'd admire the newbies only to find the knife fish had been slightly attacked and was a bit marked, but the arowana had been demolished, scales torn off and everything.
I seperated the arowana in a hospital tank but it died very shortly after.
So my advice is no, but that is on one bad experience i have had, it does all come down to the individual behaviour of the fish you own, so you have to take your own risks. I certainly will not be mixing these fish again.
In fact the oscars are now in with an 18 inch tiger shovel nose and a huge pacu, and theyre not so boystrous now! And i plan to go get another arowana and hopefully this will not ever happen to me or anyone again!!
