Damn i miss mine. He killed my 8" florida wild caught pbass but he was still the coolest fish iv ever had. Never seen an aquarium fish hit food that hard. its like a combo of a salmon and rainbow trout on steriods. but they need more room than i could ever house. Goodluck with him he's already looking beautifully goldy.
Arrowanas seem to work well as do some silverdollar type fish. no idea why mine was only aggressive towards other agressive species. he let my arro swim and eat but wouldnt ever let my pbass eat or even do more than hover over the bottom and would even attack my 14" Leiariusmarmoratus. Mine was telling me he wanted his own tank but goodluck to you and your Salminus.
These are definatly up near the top as favourites of mine. I had 2 that I lost due to a tank breakage. The best word I could use to describe these fish is machines. they have purposeful presence with a real don't mess with me attitude.