There is a lot of confusion over the eventual size of black arowanas, some say they top out at a little over 2 feet while others say they will get just as large as the silvers and can grow to 4 feet plus. I tend to lean towards the side of them being one of the smaller arowana species, ive yet to see one reach the 30 inch mark, as long as the tank is a minumum of 6x2x2' then your arowana will be ok in there for many years if not life though a tank of 8x3x3 would be the optimum size for any arowana species.
Before you buy your arowana i'll offer you a little advise, many of the black arowanas that come into the trade now still have their egg sack attached, if the one you are going to buy has this do not buy it, also before buying the arowana ask to see it/them fed, arowanas are always hungry and should readily accept the food, if they do not feed or the lfs owner/manager refuses to feed them do not buy them. Try to buy the biggest and strongest one from the tank, small arowanas are very delicate and can be fussy eaters so there is a very high rate of mortality in fish under 6"