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It's part of their natural color. Nothing to worry about. There are 3 different Tetras that are clumped together and called Rummynose Tetras. What you have are these: Petitella georgiae ( common name: False Rummynose Tetra. ) The other 2 are Hemigrammus rhodostomus and Hemigrammus bleheri. The difference between these 2 is the amount of red in the head and it's position.