Clean water can usually prevent these kind of problems but your going to see a bad ammonia problem if you completely changed your filter. Thats definitely fungal though and needs treating. How long has your tank been set up?
If anything a uv sterilizer should prevent it and not cause it.
Youre going to have huge problems, if youve completely removed the old filter than youve lost your cycle. If you want to save your peacock you now need to do large water changes daily to keep the ammonia down. Just as important is the fungal medication.