If you're using Foz Down, or for that matter, any additive that requires skimming to remove after its done its deal, be sure that your skimmer is actually, physically skimming. Just returned home after 5 days away to find all of my pociliporas (including my prized 12+ year old colony) have bit the dust and a few non-pociliporas have started as well and everything has browned out. They were looking kind of sketchy before I left but I had no idea they'd be this angry over whatever is in the water.
I'm not saying the product is bad, just that if you have an additive that really relies on a skimmer, make sure its working. It would seem my skimmer is "working" but not really doing much skimming (or something, I haven't looked at it yet. Euro-Reef is not my friend right now
).
Off to do multiple waterchanges and look at skimmers online (and only dream of actually buying one).