57 gallon
3 x barramundi..................4 - 5"
1 x electric yellow............. 2.5"
2 x severums.....................2"
1 x rainbow shark(i think)..3"
1 x lifalili jewel...................4"
1 x oscar............................6"
These are some really really aggressive fish. They will tear each other to shreds eventually in a 57g tank. The Oscar alone might take the entire tank, but could possibly tolerate a Jewelfish and a Rainbow Shark. Water quality is my utmost concern when suggesting that.
Barramundi bothers me, because, as Crocodile Dundee said, "[They are] a bloody big fish." Plus, they are illegal to own in many countries (maybe not in Australia) and are among the most aggressive fish out there, not to mention the fact that they like to be loners, as most arowanas tend to be.
If the electric yellow is a Labidochromis spp. or another Malawian cichlid like I suspect, it may need a different tank as most of the rest of these fish would do best in much softer, much more acidic water than one of the Malawians.