bummer man, i got me some velvet recently too. i also think i brought it in via coral, but maybe it was more due to my subpar QTing practices, can't really ever pinpoint the source of this stuff
idk where your tank is at coral wise, but mine was still newer so removing all of the rocks wasn't the end of the world. still sucked, but fallow with proper qt is still probably the best way to go. if your tank is loaded with coral, then i'd consider the h2o2 dosing regime like hersheyb advised
sounds like you're already familiar with velvet and how it works so i'll save that
what i did was remove all fish and gave them all 30 min 150 ppm h2o2 baths en route to various QT tanks. for 7 fish i probably wouldn't use just a single 20g. i used some tanks i had hanging around + some heavy duty storage totes. you can get them at home depot. i then ramped up copper fairly quickly in some of the tanks and then had a wrasse only tote so i could ramp that slower
out of 25 or so fish i ended up losing 4 total. 2 in the DT due to velvet, 1 post copper (pintail fairy wrasse that i think crashed into something randomly) and 1 during copper that i think may have been an aggression issue. i did copper power and tried to keep it closer to 2.5 than 2.0, but i did have it venture down into the teens. i also had one tote venture up to around 2.75, but i wouldn't recommend that. i didn't know this, but copper is like salinity. it doesn't evaporate, so if you don't topoff consistently it'll rise. but stuff can also absorb it. ime using marinepure spheres and other biomedias and even a decent amount of sand, it doesn't leech much at all, but that might not be the case for you
it was also advised to me by humblefish that bacterial infections were likely so the best is to dose anti biotics into the qt tanks, but i only had a few doses of spectrogram hanging around, and with like 4 tanks that was gonna last like a couple days at best. so what i did was every other day i added the abx and laced it to the frozen fish food with seachem focus. i never encountered any bacterial issues, so i guess it worked. hard to prove that, but i have had issues with bacterial infections using copper in the past
if you go this route try to be proactive with what you'll need. i ran out of hanna copper test packets and had to order those, it's recommended to test copper daily, so those go quickly even if you start off with like 10-20. i also saw the abx issue looming so i ordered what was available. spectrogram was not available so i ordered a couple sets of furacyn
https://www.americanaquariumproducts.com/Aquatronics.html
my post 2 weeks of therapeutic copper plan was to transfer all fish into 2 more large totes, but this was dumb by me as i couldn't actually observe the fish, which seems important for post copper observation. i ended up grabbing a 40 breeder from petco (dollar per gallon sale was going recently) and i had an old 55g tank hanging around from my early hobby days before i knew what a sump was
good luck with whatever you choose. humblefish has a site you can ask him questions if you want more expert guidance
Did an ro fw dip on the wrasse and kole tang.
