An OPAH is a large, colorful fish that is found in tropical and subtropical waters. Tetras are small, colorful fish that are found in freshwater aquariums. A tetra is a type of small, freshwater fish that is often brightly colored. ERAS are small, colorful fish that are found in both freshwater and saltwater aquariums. Neon tetras are small, brightly colored fish that are found in freshwater aquariums. All of these colorful aquarium fish can add a vibrant and lively atmosphere to any tank.