If not for the other fish dieing I would guess general systems failure caused by old age however...,
What it sounds like is a combination of a serious kidney infection combined with a secondary infection of the gills by either gill crustaceans of sporozoal cysts.
The Crustacean infection can be treated with either de-los and methylene blue, or any strong formalin/malachite green mix such as forma-green or clout. If it is sporozoans then there is no listed treatment though again formalin/malachite green has sometimes worked (rarely). Disecting one node (sac) will reveal either a tiny multilegged critter (crustacean) or a hollow sack of fluid (sporozoan). For the kidney infection try either erythromycin or TMP Sulfa.