Consider how apex predators live in nature.
A pack of wolves may require a territory 50 square miles.
1 tiger may require the same 50 mile area.
Although a 2 ft long apex fish predator may not require 50 miles, ...
where I collect fish, I might find a 12" Gobiomorus to be the only apex predator of its ilk in a stretch of river the size of a city block.

What is the size tank they are in?
Why should a P-bass or Arowana be be any different? why would any apex predator species of fish be expected to peacefully share even the largest tank?
Below is the section of river where the goby above was caught.

In this section of river we caught tons of schooling tetras, many plecos, a few non-picavorine cichlids, but only 1 Gobiormorus.
We cast our nets in all the visible locations of the photo, yet only the 1 apex Gobiomorus.
Sure we may have missed one or 2, that may have been there in that amount of space. but....this might suggest
in a certain amount of space, expecting more than 1 such apex to live peaceably may be stretching that spaces reasonable limits.