I'm wary of live feeder fish bought from pet stores - they are crowded together and kept in fairly unclean conditions (as a rule). If you were raising your own guppies, that might be okay - although I have heard conflicting information. On one hand, guppies are often recommended as safe feeder fishes because of their presumed to be low thiaminase content. Some people claim that guppies contain thiaminase producing bacteria, however I have never read any scientific study confirming this.