Guys, this is coldwater fish thread so obsviously he don't ask for tropicals. I would do some darters such as Rainbows, Swamps, Johnnies or Fantails and few killifish such as banded, golden, blackspotted, bluefins etc. Or if you want some predatory fish, then Mudminnows, Pirate Perch or Sculpins would be your bet. If you want a solo or two fish then there are few smaller sunfish species such as Banded, Bluespotted, Orangespotted or Bantam. Melanistic Mosquitofish, three-spined sticklebacks, four-spined sticklebacks and brook sticklebacks may be aggressive but they still makes a great fish for a 10gal tank. Madtom is another good choice as they stays smaller unlike their cousin, bullheads and catfish.