Sorry for your losses
It is hard enough to see one fish kick it, but to watch your tank go south, that's rough.. It does sound like velvet, or at least a velvet like pathogen. If you do a forum search you'll find a few people here who have had it and have detailed their experiences with it. If it was velvet or something similar the decline and death of the fish could have easily brought down the fragile biosystem
Unfortunately you can not test for Velvet. In hind sight, the only thing you could have done was to QT, observe and possibly treat your clown tang. I realize that there are two camps on the subject. Those who don't QT for good reasons, and those who do QT for good reasons. Deciding which camp is "better" is up to the hobbyist.