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Red streaks *can* mean some sort of infection or bad water like high ammonia. More info is needed before we can take a stab at this ....
water parameters, tank size & inhabitants, behaviour & feeding, poop & colour, fins clamped or open, which fin? other spots, dots or lumps? a pic might also help.
need more info. like jsoong said could be any number of things. if youre water parameters are fine, you should change the water and add 1 tbsp salt per gallon and observe further. often this is caused by high nitrates, but need more info to be sure.
I agree on checking the water parameters. Especially Nitrates, if you haven't already. Just in case your wondering, the salt will reduce the Nitrate uptake in fish.