It is approximately 47 US gallons, IMO one oscar would fit. The two most common types of tanks this size have footprints of 48"x12 1/2" (120 cm x 31.25 cm), and 48"x18" (120 cm x 45 cm), the wider of the two would support an adult oscar. Neither tank would support even a 1/2 grown arowana or Fla. gar.
Micheal, you would be better off going for fish that do not exceed 8" (20 cm) in the narrow style tank and 12" - 14" (30-35 cm) as adults in the wider style, it depends on the type of fish, in general inactive or slow moving fish require less space. Also if you go for an oscar and pleco mix make sure you use filters rated for around twice your tanks volume, they are poop machines.
Do a bit more readind and you will find fish that suit your tank and your taste.
If you have the wider style tank you might consider replacing the gar with three Boulengerella maculata (pike characins) pic by Peter Lutz, or three Ctenolucius hujeta (rocket gars) pic by P. Marshall. Replace the oscar with something a bit smaller like a Cichlasoma octofasciatum (Jack Dempsey) pic by Johnny Jensen, or something even a bit smaller. Then add a couple of interesting catfish like the Syndontus multipunctatus (Cuckoo catfish) pic by D.J. Riedssell, and a Ancistrus temmeckii (Bristlenose pleco) pic by Greg Steeves.
There is no smaller equivelant to the arowana.
The above mix will give you an attractive and unusual tank population but is still (barely) within what a tank that size will support. It will still need heavy filtration.
