im sorry to hear you lost one of your geckos. im not an expert or anything, but you mentioning a purple spot reminded me of a skin infection leos can get. they develop patches of black/brown on their belly's as a result of being kept on badly soiled substrate or if the substrate is too moist. my book recommends separating them in a clean, simple tank and to treat the infected area with Betadine, Neosporin or Polysporin. other then that, it could be impaction. though i don't know what symptoms they show other then lack of appetite and weight loss. maybe someone else on here can tell you. it wouldn't be a bad idea to get your others checked out by a vet.