velvet is a parasite and all salt water fish are able to get it. It attacks the gills so even if you don't see it on the fish it is there. If your tank has velvet every fish you add will likely die. Velvet can kill an entire tank very fast, much faster than ich. You can take all the fish out and treat them with copper but you can't put them back in the tank until the velvet dies off. For ich it is 72 days I am not sure about velvet but you'd be safe at 72 days. This is why you must always quarantine fish before you put them into your tank. One sick fish can kill all the others.