Did you drip any of the water in it? Because contamination occurs very easily when moving stuff from one tank to the next. And, leaving the tank fishless for 8 weeks doesn't mean velvet won't come back. People have had it reoccur anywhere from weeks, to years later. It's a pretty horrible disease. They make a two part solution for it, that is reef safe, and has had some success stories accredited to it. I forget the name of it, but looking online should help you find it. I had it in my 55. The only way I got it to start coming back was to take down the tank, and clean it with bleach, acid bath for the rocks, and all new sand/water. It succcccks :(