Ich/velvet in a reef tank is a b*tch. I've had both happen, even with quarantining. Even letting a tank sit fallow for 90+ days afterwards. Considering how long it can lay dormant in a fish, 30 days quarantining doesn't really do much. Treating during that 30 days will increase the odds, but even then a lot of the parasites are bullet proof until they leave the fish.
One thing I have found, that I use long term as a preventative, is ick shield food. I put some in a tiny bit of water, turn it to mush then soak mysis shrimp in it. Then during the day 3-5 times a day, I drop tiny minute amounts into the tank and let the fish eat it. They seem to gobble it up like this. Even my copper band butterfly. I do this for three weeks. If I see so much as a spec on a fish I begin treatment again. So far, this seems to have worked. Although without correct diagnostic tools I cannot even say for sure if the fish ever had ich or marine velvet to begin with. And thats the biggest issue with treating disease. Without proper diagnostics we have no idea what it is the fish has. We can observe and make a logical guess, but it's still just a guess.
The more I read on ich, velvet and other nasties the more I am convinced there are people who have had disease in their tank and people that will have disease in their tank. You can take appropriate steps to prevent it, but it's not fail safe.
Didn't mean to hijack your thread. It just breaks my heart every time I see people lose stock to this crap and I am left wondering how, in this day an age, we cannot have some type of in home diagnostics, a vaccine, or a 99.9% treatment that's reef safe, invert safe, and even fish safe.