I found this which was written/posted by Lion_Crazz. Not sure if it applies to guppies as well, hope it helps:
47. NEVER FEED A FISH FRESHWATER FEEDER FISH. Freshwater feeder fish cause saltwater fish to get fatty liver disease, a disease where the saltwater fish’s live gets so large that it can no longer process any food. Usually, goldfish, rosy reds, or guppies are used to feed lionfish, groupers, triggers, wrasses, puffers and other aggressive fish. This is not a good practice but one that will eventually lead to the death of your fish.