You can safely perform hyposalinity in your tank, with no ill effects to your fish, your tank.
Take a look at the FAQ Thread at the top of this forum, and the post on ich which details the procedure, as well as provides info on the parasite itself. It does sound like ich, not velvet.
For hyposalinity, you need a refractometere or a quality glass hydrometer.
If you need a reference, take a look at the Diseased Fish Thread, also at the top of this forum, and look at the pictures there of fish with ich.