[RIPTANK]

Check your pH again. I'm bettin' it's your pH! Pick up a good pH test kit. Preferrably the Instant Ocean for about $15. Very accurate and easy to use! I had the exact same problem. My fish all dropped dead within 12 hours and were covered with a slime or mucus coating from complete stress and my LFS finally pinned it down to the pH.

Or better yet, take your water to your LFS and ask them to test the pH for you. If it is your pH then pick up some ReefPure pH buffer. Very good stuff and only about $12-$15. Be sure to mix it with some of your aquarium water first before putting it into the tank! Don't just throw a teaspoon of it directly into the tank or you'll burn your corals.

I was scratching my head 'cause my salinity/sg, temperature, ammonia, nitrate and nitrite were all reading good. Then finding my fish dropping like flies and covered in a white film of mucus. The cause of the mucus is from the fish over producing their slime coating to protect themselves from the acidity of your water. This quickly exhausts their little systems completely stressing them out and kills them. So chances are your pH is reading 7.9 or lower where it should be at 8.2-8.5.

I lost 2 blue spot jawfish, 1 percula clown, 1 yellow eye kole tang and a 6 line wrasse to dangerously low pH. All died from the same symptoms: Gasping for air, covered in a white mucus membrane and completely pale.