Tank smells powerfully of rotten eggs

[asylumdown]

This appears to have starts this morning, to the point where the whole house smells like sulphur. I am at a loss to explain this, corals are all happy, fish are fine, snails and crabs are motoring about. The only thing that's died recently was an elegance coral, but it's skeleton has been free of tissue for several days. I was having an issue with my r/o water smelling a bit off, so I cleaned the unit and replaced all the cartridges, and now the R/O system is working perfectly. I also siphoned about 5 gallons of water out of the tank and took it to another room to be sure it's the water, and it's definitely emitting a potent rotten egg smell. I am getting a little worried...

Any ideas? Even a dead snail couldn't make 375 gallons of water smell this bad.