Firstly all arowana's grow BIG so unless you have a minimum of a 180 gallon tank (6x2x2') then dont even consider buying one, even the smallest of the arowanas (which are the Asian arowanas in captivity) will grow to around 2 feet and silver arowanas can reach up to 4 feet, the three other species (black, jardini and leichardtii) will also grow to a minimum of 2 feet and up to 3.
All arowanas are fiercely predatory and will eat any smaller tankmates they are also very aggressive toward other arowana's and other surface dwelling fish.
Arowanas are sensative to poor water quality so regular large water changes and large filtration systems are needed for their care, nitrates should not be allowed to exceed 50ppm.
To keep a arowana healthy feed it a diet of floating carnivor pellets/sticks and crickets or locusts, some frozen meaty foods like mussels, cockles, shrimp and fish can be fed once or twice a week but no more as these foods can lead to fatty deposits forming in the head and behind the eyes causing deformities, AVOID using feeder fish which are a poor source of nutrition, are usually very fatty and can introduce diseases,