Bacteria are always there, most don't need be imported, and most are benign, unless there is one (or a few) stress factors working to compromise the fishes immune system, then, normally benign bacteria can become problematic.
Stress factors can be anything from too hard water for soft water fish, (or vice versa) a bullying tank mate, temps out of line, or simply elevated nitrate from too few and small water changes.
I find the last one, is the most common reason and most responsible, because as fish grow they produce more waste, so need more and larger water changes. If not done, cause conditions for the bacteria to thrive becoming internal vectors in the digestive tract
Stress factors can be anything from too hard water for soft water fish, (or vice versa) a bullying tank mate, temps out of line, or simply elevated nitrate from too few and small water changes.
I find the last one, is the most common reason and most responsible, because as fish grow they produce more waste, so need more and larger water changes. If not done, cause conditions for the bacteria to thrive becoming internal vectors in the digestive tract