A '0' reading of NitrAtes is usually a sign of an uncycled tank. '0' Ammonia and '0' NitrIte are great, but if there is no indication of any NitrAtes then your bacteria are either not there or not doing there job.
You will have to post some more information about your tank and its filter... how long have you had it? Have you 'cycled' it at all? Do you currently have any fish in there? Do you have your own water test kit?
Terry.
PS.. but judging by this ---> http/www.fishforums.net/index.php?/topic/316534-male-guppies/page__p__2623502__fromsearch__1#entry2623502 i would say you need to do a little bit of research into the 'Nitrogen cycle for tropical tanks and filters'. Everything you need to know can be found in the beginners section.