PROGRESS! i might have found it!
here is someones similar sounding case (quite similar...)
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I just added three peppered cory's to a 6 week old tank (20 Gallon). One died the 2nd day and it looks like he'd had fin rot or something like it for a while. The second seems to be thriving hapily but the third has these red spots that are getting bigger and redder. They seem to be right where his pectoral fins are attached to his body. I don't think it's his gills but I could be wrong. I've tried Melafix for three days and the spots are even worse. He is swimming and eating, but I'm worried he might die like the first one Any ideas
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and a response which doesn't seem as accurate but might help anyways.
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Haemorrhagic septicaemia is 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 toxic ammonia and/or nitrite levels, 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 observed on the fins or red spots on the body are 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.
you need to maintain your water quality and reduce/eliminate both ammonia and nitrite. there's little point in medicating these fish unless you eliminate the cause first. otherwise it just won't work.
you do not have a cycled filter and thus, the toxic nitrite levels. moreover, corydoras catfish are not hardy to these toxins and should never be insstalled in an uncycled tank. i believe you're seeing the results now.
having only one or two fish out of several 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.
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