I think you're just taking things too fast with your tank. I've had my tank up and running about the same length of time and I don't even have a quarter of the livestock you do, and I'm still putting off making my next purchase (which I've decided is gonna be a mandarin goby... YAY!). Its like a growing child... you can't expect it to be able to do everything you want it to do from the get go. Give it time to mature and settle between additions.
The brown spots on your foxface are probably just because of stress. My yellow tang had the same problem his first week in the tank, before he staked out a home territory to sleep in and feel safe. Most likely that'll stop... if you want to help him, try making a little cave or sticking a piece of
PVC in the tank for him to feel safe in. He'll be fine.
Edit: Oh yeah... I would not go ANYWHERE near additives in your tank, especially when you're having problems with your livestock and you don't know why.
