one tank isnt going to do the job!, i suggest atleast three, one for the females to become inseminated, which is where the adults go, this is o'caorse after you have a secure strain, one for maturing the fry to adults, and the other for breeding specifics, such as a good looking father breeding to a stong bodies good colour sister, to see if you get a better colour in the female or a stonger body in the male etc etcl, i use atleast 2 foots so as not to stunt the growth of my guppys
if you need anymore help, i suggest you post it in the livbearering forum
Cheers Shae