Sorry for your losses as well, unfortunately a select few fish can become resistant to velvet (wrasse because of the thick mucous coats, the fact that they sleep under the sand and/or in mucous cocoons— and gobies to name a few occasional possibilities) but the issue is if you ever plan to keep other fish, or really much of anything long-term, they’re going to continue to reinfect your fish. Even new additions that are wrasse or gobies will likely die — by the time we receive fish through he distribution system their immune systems are so weak from transport, constant medication exposure at subtherapeutic levels, poor water quality, and perhaps worse of all they’re often not fed hardly anything, if at all, so that they don’t soil the shipping bag and save costs.
It’s a tough reality. I managed velvet in my wrasse tank and even had a blonde naso tang that built the resistance — for 6 months though I slowly lost fish due to gill infestations beyond sight to velvet and eventually the tank wiped out with a power outage event. No one stood a chance with a low level velvet infestation, it lies in wait for the slightest lapse in full immune response, living within the gills of the fish out of sight if they’re “immune” (in reality, this is just a resistance). 95% of fish inhabitants though will not be able to do this, and it wasn’t the ethical thing to do.
Eventually I removed and treated the blonde naso tang. I’ve never looked back my fish are much happier, far fewer die, and they look and behave MUCH better!