Quick answer — The Seatbelts performed 2 anime themes, including "Tank!" from Cowboy Bebop, "The Real Folk Blues" from Cowboy Bebop.
The Seatbelts performed the anime theme songs "Tank!" (Cowboy Bebop), "The Real Folk Blues" (Cowboy Bebop).
The Seatbelts's first listed anime theme is "Tank!", the opening of Cowboy Bebop.