An Australian or Asian arowana would suit your tank best as these are most likely to remain under 2 feet (although both species are capable of growing to 3 feet in some cases) and are fairly hardy, but both come with draw backs, the Australian (Scleropages jardini) is highly aggressive and is unlikely to accept tankmates in what is a relatively small tank and the Asians (Scleropages formosus) are extreemly expensive with a 6" green (the cheapest varient) likely to set you back at least £200 and more for redtail golds, baja reds and golds going up into the multiples of thousands.
Blacks are extreemly sensative and are not really recomended for first time arowana keepers and silvers (the most common arowana) grow far too large for your tank easily reaching 2 feet in length within the first 2 years and up to 3 feet plus over the next 3 years after that.
A diet of meaty frozen foods (mussel, prawn, cockle, clam etc), arowana sticks and live crickets shrimp and earthworms will keep your arowana in good health and provide all the nutrients it needs though young arowana's can be fussy and may only take live foods to start with.
All arowana's need pristine clean water so double filtration and 30% weekly water changes are recomended.