12-09-2009, 07:05 PM
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The flashing is usually due to parasites (like ich) or water problems (ammonia, nitrite, nitrate, fluctuating pH and hardness). In this case it may well be ich. Fish that are stressed (as they are when netted into a bag and then introduced to a new/different tank) are very prone to contract ich. However, healthy fish in good water (quality and parameters) will often fight it off. I notice this now and then in my tanks especially after new fish are introduced, but very rarely do I see spots because the fish fend off the attacks. I have had fish with 2 or 3 spots and never treated the tank, and in a couple days it is gone and no other fish show any signs of it.
Keep a close eye on the fish. As you have it in a separate tank, it will be easy to treat if ich does appear. However, corydoras are sensitive to all medications and I would not medicate except in extreme cases. Again, if he is healthy, well fed and not stressed, he should be able to fight it off.
Clean water, suitable water and good tank maintenance is the best prevention.