Spirulina is a blue green algae, originally found in volcanic lakes. Spirulina is known for its ability to increase resistance in fishes. This helps fishes fight infections.
It has raw proteins and almost all essential vitamin. It is rich in fatty and amino acids as well.
Spirulina also has color enhancement properties.
Fish foods which contain Spirulina as a staple are available. Check that that foods which claim to be spirulina based have spirulina in excess of 20%.