Awesome - now we've ruled out something. That said - can you give more information - did you get your fish from the same place - did you QT them, did you treat them? Did you leave the tank fallow for let's say 76 or so days - before adding new fish? Are you using RODI water? Is there any other toxin in the room (I discount this - as well as stray voltage) ?
That's Brooklynella. You can try Ruby Reef Rally Pro, but that doesn't always work. There really isn't any other treatment you can use in a display tank.