[i have crabs]

im sorry too, its really crappy to have things go sour on you to the point of things not making it. ive never read that much about velvet but from what i do know ive always thought it was similar enough to ich that i dont need to be an expert. i think your doing the right thing by coppering the fish in a qt, the assumption that copper kills easily is something i dont pay much mind to since i also use the seachem stuff which is a non cleated copper and much less toxic, normally i do the recomended dose wait the 24 or 48hours they recomend on the bottle and dose it again, and if i think the fish look in bad shape after a day of that ill check amonia/nitrite/nitrate and if thier ok ill dose a 3rd time and leave it at that concentration, the first time i ever did this i used a copper test kit and tried to be safe and ive never bothered since,
about 4 years ago i lost a tank full of large fish to ich only 2 or 3 fish made it 1 died since then,now every fish i get dosent matter from where goes into a qt and is treated with at least 2 doses of copper for 1-2 weeks and ive never seen a spot of ich in any tank since,

again im sorry that you ended up having to lose fish also but please take away the habit of qt'ing every fish from now on.