[Delphinus]

Hi, I just posted in your journal thread but I'll post again in here for the sake of completeness.

I suspect something pathological in nature - either a form of boring algae (are the skeletons greenish after an RTN event? If so, it's a kind of single-cell algae that bores into the skeleton), bacteria, virus, or fungus.

If that is the case then essentially there is no solution short of running a fallow tank and hoping it eventually dies back to the point it can't come back.

It thus may be easier, emotionally, financially, practically ... to simply shut the tank down and start over. (I probably should have done so a long time ago myself, but pressed on due to having fish and other livestock that continued to do well.)

I know it's not much of a suggestion, sorry. It's just that there's really nothing obvious regarding the science that we can do (ie., check parameters, look for obvious stressors, etc.).