Change = Stress
A change like that can stress your fish in the beginning which can lead to a weaker immune system.
Ich is often in the system without being a problem. It does wait for stress to help it become a plague.
It is not likely that the light itself caused the pathogen to reproduce quickly.
Even with all of that - it would take a couple of days for there to be a problem. Make sure of two things:
#1: that your fish were not infected already but lower light levels made it hard for you to see.
#2: that what you see is not just something else on your fish.
While I do believe you have ich, I also believe the fish were already in the midst of an outbreak and that the timing of the light was a coincidence.
How long have you had the fish? Did you quarantine them?
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