Help!! Need some advice from coral keepers!!!

Corals have nothing to do with it. I would bet eels are getting stung by lion during feeding time.

 

First and foremost, I know as much about eels as you do about corals.

I have kept dozens of coral species over the years and can trace 0 fish losses back to corals. They do secrete mucus from time to time, and I've heard of chemical warfare between different Corals but have not heard of that effecting fish.
I've had coral die offs before with no effect on fish. That does not mean it can not happen however.
Honestly, I would look to something else being the cause.
What was in the 10 gallon tank you added to the system? Could the lion fish be responsible?

 

I'm not sure on the truth to eels being immune to a lionfishes venom but a friend of mine kept his eels with a large lionfish for years with no problems at all. The eels that died, were there any markings or anything on them that would indicate something? Also, whats the approximate age of the ones that died? I know they are a fairly long lived animal. What are you nitrates at? Have you been checking to see if there's been any kind of major spike?

 

Youre right about the tang showing signs of "stress" or illness before the eels. Gradual changes or increases in parameters like your temp shouldn't be an issue, they should adapt fine to that. However if your heater stuck on and caused a rapid spike in temp I could see that being hard on them. Wish I could be of more help, I'm a fan of eels too. This is certainly an odd scenario going on

 

Do you run carbon on your tank?

 

I have no clue about eels but am going to throw this out there. Since you added some things from the 10 gallon tank such as live rock. Is there something that could possibly be leeching out of the new live rock/corals that doesn't affect fish or corals but has an adverse effect on eels? Have you changed food sources, or added something new. A friend of mine lost his entire show tank when a customer threw some type of metal into the sump. My thought is to think about anything that you may have changed in the last 30 days.

 

I would agree, the issue seems like it should track back to one of the items that has been added in the last month.

Another thought is what food are you feeding them? Anything change with that in the last month? You did say all the eels appetites have changed, is it bad food that took out the two and the others just haven't had as strong of an affect from it?

 

I would put money on the lionfish being the culprit.

 

Well if you've gone this long without ever running carbon I wouldn't want to have that create any new catastrophe. I am a big believer in it though. If there is some contaminant in the water that the eels cant handle chances are carbon will pull it out. Not something you have to run full time but if you have a reactor you could pop onto the tank with carbon and run it for a day maybe it will catch whatever it is.