I have four SA arowana 27-30" long in a 96x36x30" tank. It is doable but sometimes seems cramped. The tank that size was much better suited for three adult arowana.
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I know some will say 1 or 2. But I have seen as many as 5 in a tank that size and they were huge.
your tanksize is good, but gonna depend if the aros will tolerate eachother
they eat alot hopefully you take that into consideration =] i only have 1 and he goes through about 10 dollars worth of shrimp every month. that dont even included all his other foods that he gets fed. but with that being said he/she is totaly worth it if i had a bigger tank id have more lol
i have 5 growing out in my 180gal and they are running out of room fast!
i would agree with brian and say 3 maybe 4 if they really like each other. i think your about 2' shy on the length and 1-2' shy on the width for any more then that.