When was the last fish addition, and do you quarantine fish? It's possible that your fish were asymptomatic, at least to your eyes, and that when the conditions were right to lower their immunity enough that the parasite was able to take hold. A lot of times the parasites will only be able to infect the gills of the fish and be able to maintain in small enough numbers to not be eradicated but also not be able to spread like they want because its host is otherwise healthy enough to produce enough of a slime coat to fend off the free swimming stage on the body. With that all being said it is certainly possible that the velvet came in with the starfish instead of on it. If any of the free swimming parasites happened to be on the starfish, rather than attached to the starfish, its certainly possible that was the route of introduction. Does your local store keep fish and inverts in the same system? I know you've already spoken to the/a store and they know that it's in your system but if it was the same store that you got everything from it may be worth it to let them know to keep their eyes peeled for any symptoms. If at all possible, it would be worth it to remove the fish from the display and get them into quarantine so that you have a smaller volume of water to treat, or if that's not feasible maybe it would be easier to remove the inverts to a quarantine so as not to expose them to the medication/treatment procedure as anything that will be strong enough to wipe out velvet will also kill your inverts/coral. Sorry for the extended post, hopefully something I said will help out. Good luck, marine velvet is a bear to deal with.
