it is better not to have any powerheads..arowana loves slow moving to stagnant water. if you are feeding your aro with prawns..be sure to remove the shells...it may hurt the aro's stomach and intestines..
for that size 3-4in he can give him pellet, krill and bloodworms. as for the powerhead, it will be better to avoid the powerful ones for such small aro as freak accidents do happens.