My Oscars let go of some slime after water changes. May be as simple as that. As for the Oscars that refuse dry food, this should never happen! Oscars are notorious for excepting anything. If all they eat are feeders then they are spoiled and sooner or later will catch a disease from a ten cent Goldfish. Get them off the live feeders as soon as possible if you want to have them for the long haul.