if money were no object, and feeders didn't count as second species...sand tiger!!!what's more monster than a fish that eats its own siblings inside the womb?
a hevily pregnant female livebearer. I don't like livebearers that much, and they naren't that interesting, but its the only lopol i can think of to the "one fish for life" part. Gotta have a lot of fish
but seriously, electric catfish. They are very forgiving, great preditors, and you can even train them to do tricks!(I trained mine to swim through rings for tubefex worms), plus, you have got to love a fish that feels like an electric fence when you touch it.