O.S.I. Spirulina Flakes are not only an excellent natural color enhancer, but the algae spirulina also stimulates your fish’s immune system to attack gram-negative bacteria. Gram-negative bacteria are known to cause a variety of diseases in fish. The result of feeding O.S.I. Spirulina Flakes is healthier, heartier and more vibrant fish. This diet is appropriate for all freshwater and marine fish.