The zooplex may have particles that are a little too large for the feather duster, but that is just my guess. Honestly, you can get away with not feeding the feather duster at all. If you feed more than once a week there is a very good chance that your water quality will foul.
Also, when feeding, don't use more than 1-2 ml (will look almost like nothing in the syringe). If you can get your girlfriend to agree on it, I wouldn't feed the feather duster more than once a month.