The bacteria that cause fin rot (primarily aeromonas and pseudomonas) are present in all of our aquariums in low numbers all of the time. It is only when a fish has a break it its immune defenses that these bacteria pose a threat. This can be caused by rough handling, water quality problems, fighting/fin nipping, overcrowding, poor nutrition, and a variety of other sources of stress.
If only one fish was showing signs of disease, then it likely isnt a water quality issue, overcrowding, or poor nutrition. If you had the fish for a long time in the same tank without handling it, then it probably isnt from transportation/handling stress. This leaves fighting/fin nipping as one of the likely causes. If that isn't it, then as previously stated it may have been some random source of stress that you may never figure out, especially if this never happens again.
If only one fish was showing signs of disease, then it likely isnt a water quality issue, overcrowding, or poor nutrition. If you had the fish for a long time in the same tank without handling it, then it probably isnt from transportation/handling stress. This leaves fighting/fin nipping as one of the likely causes. If that isn't it, then as previously stated it may have been some random source of stress that you may never figure out, especially if this never happens again.
*To bbortko :