velvet is one of the most dangerouse parasite to treat.
it can, and often does, wipe out an entire tank.
the best treatment is a copper based medication (note; will kill any snails, shrimp etc). I have had sucess with waterlife cuprazin, which is a copper based med aimed at the marine side of fish keeping.
just follow the dosage on the bottle.
the other important thing with treating for velvet is light,
the parasite is stronger in light, so during treatment a total blackout will help speed things along.
I wrap the tank in a heavy blanket or duvet, ensuring there are no chinks for light to get in, obviously turn off the tank lights too.
this should be kept in a dark room as well, so curtains drawn at all times.
no peeking, no feeding. the only time you should access the tank is to add more medication.
the blackout should be for the first 3 days of treatment and the following two days (if using cuprazin) should be unwrapped but with tank lights off.