Hi
Channa aurantimaculata get to about 40 cm / 16 inches
Channa pleurophthalma get to about the same and requires a large aquarium just for the speed they can move.
Your problem with your tank is the 15" depth so I my self would go with a dwarf or a collection of dwarf snakeheads.
Channa gachua It is one of the smallest species of snakehead and has a length of about 20 cm
or Channa bleheri also about 20 cm / 8 inches
due to there size they are the most suitable snakehead species for your aquariums and you can keep them for life.