You don't hear about it as much in Angels as you do in Tangs, but maybe HLLE
? What are your parameters? What kind of filtration? HLLE
does not have definitive causes, but seems to be linked to stress, poor water conditions, poor nutrition, possibly stray voltage and even using carbon filters. Did it start small and grow or was it normal one day and big hole the next?
Tangs can get territorial, so one of them might have got him. That could be it if its an overnight sort of thing.