Yes, UV's are very effective against Velvet due to the trophonts small size.
There is a lot of literature out there about how to configure a UV's flow rate. If you have too low of a flow rate for the tank volume or too high of a flow rate through the UV or an old bulb they will not be effective.
This book will explain it.
https://www.goodreads.com/en/book/sh...ms_Engineering
Cheers,
Tim