sounds like it may be Haemorrhagic septicaemia caused by one or more of the bacterial class of Aeromonas, Pseudomonas or Vibrio. these bacterial species are common in many apparently "healthy" aquaria as latent infection waiting to happen. happen it does when fish are in poor and/or stressed condition for some reason, which can include overcrowding, moving, rough handling, inadequate/innapropriate diet, temperature fluctuation, wrong pH or pH fluctuation, build up of dissolved organic compounds --- the list is practically endless.
the red streaking you observe on the fins is the result of haemorrhaging of internal organs and accumulation of released blood in more external tissues of the body such as fins. other symptoms which may occur as the disease progresses include ulcers.
there's little point in medicating these fish unless you are willing to eliminate the cause first. if you do otherwise it just won't work.
having only one fish out of two contract the disease is not uncommon -- fish vary in their resistance to pathogens. it will not last. septicaemia is extremely infectious and the other fish will succumb soon. the result is always death.
treatment: Furizolidone at 20mg/liltre as a continuous bath for 7 days. complete water change -- 48 hour rest period. repeat treatment. Nifurpirinol at 0.2mg/litre as per above. Oxytetracycline at 75mg per kilogram of fish (yes, you have to weigh your fish) with food.
