Absolutely do not medicate the main tank. Get the fish out to a QT
and medicate them in a bare bottom tank . You can use pvc
elbows for them to hide. Leave the main tank void of fish (inverts are Ok, not hosts) for at least 4 weeks, 6 is better. The parasite will go through its live cycle and die off without hosts. If you don't remove the fish hosts, it will only get worse.
I went through a battle with velvet, effectively erradicating it with the use of Cupermine. I tried hyposalinity to no avail, copper worked great.