With zoa pox, generally you'll slowly notice some polyps staying closed. If you don't do anything, you'll notice more and more zoas close up, and you'll notice what look to be white or yellow puss pockets on the zoa, usually around the head, but sometimes further down on the stalk. If you STILL don't do anything, you'll notice the affected zoas start to shrivel up and dissapear, but again, this isnt an overnight process.
The bad thing about furan-2, is that while its GREAT in fighting zoa pox, it will exacerbate zoa fungus. It has something to do with the fact that the furan specifically battles the pox, which is a bacterial infection. It is thought to kill some random beneficial bacterias as well (not nitrifying bacteria) and apparently the lack of beneficial bacterias on the zoanthid colony leaves the colony much more susceptable to fungal infection, temporarily.
I'd try 3 fresh water dips, 1 each day for 3 days, and I'd use ro/di water thats been temp and pH matched, and 25% hydrogen peroxide.
1. Swish the zoas around in your tank so they all close up before you remove them, then do the dip for between 10 and 15 minutes.
2. Once you're done with the dip, swish the colony around in a cup of saltwater to rinse it, then put it back in the tank.
oh... i'd also cut away some of the zoanthids that are doing real bad... best to lose some of the infected ones and save the rest of the colony instead of losing the whole colony.