Feeder fish are fine. Just not Goldfish or Rosies. Guppies and Mosquitofish are healthy.
i would look at hujeta gar / needle nose gar
they are false gar but they are pretty aggressive when it comes to feeding![]()
+1 i had hujeta/rocket gar before. They can only reach 8"-10" maximum, so 20/30 gal would be fine.
as you mentioned you're looking for a small aggressive fish, this guy eats rather large fish for its size (larger than its head)
it is also hardy and very easy to keep. Also, add a bit of salt to ur tank as these guys love saltwater