As far as I know, if you choose to medicate your fish (especially dose in water column), you should turn OFF UV filter,
due to some meds are light sensitive and UV will break down the meds molecules (being destroyed without getting into your fish)
UV filter is very useful though, but for "preventive" imo.
Below copied from Seachem "Metroplex" homepage;
Directions
Dosing in Water
Remove all invertebrates - these are extremely sensitive to medication.
Turn off UV filters, ozone filters, and remove chemical filtration like MatrixCarbon™ and Purigen™.
Use 1-2 measures (included) per every 40 L (10 US gallons).
Repeat this dose every 48 hours for up to 3 weeks or until symptoms disappear.