Don't take this loss on yourself. More then likely it was not your fault. If it was your fault, we all make mistakes. First it would be best to see a picture of the enclosure. It could of just been a bad frog... It might of had parasites, we don't really know. If you are really determined to find out what was wrong you could get a fecal sample done to see if it had parasites. You saying it was extremely skinny makes me want to say parasites, but I'm not a scientist, so I don't know. Once again, don't take this hard. It happens to all of us eventually, just keep getting on the horse. It seems like you know what your doing pretty well, it was probably not your fault. Don't get discouraged.