Misgurnus anguillicaudatus, also known as the Pond or Dojo loach. These cool-water tolerant loaches are a classic tankmate for Goldfish. They earned the name "Weather Loach" because they can allegedly sense barometric pressure changes, and become more active when rain is imminent. They can be kept alone but we strongly recommend having at least 3, as they are much more outgoing and comfortable when kept in groups.
These are young adults around 4" long.