Not everyone is a fan of snails, but there’s no denying the benefits they bring to a freshwater aquarium. Aquatic snails are fascinating creatures that bring color to a tank and help keep it clean.

Leeches might seem like relics of a bygone age of medicine, but they haven’t outlived their usefulness to people yet. The freshwater leech Helobdella austinensis can eat its weight in snails every day ...