well, it depends what type of filter u r running
i have a 10gal tank, and i use EHIME 2215
i have more than 30 grown up guppies and more than 50 baby guppies(under 1 month old)
they r all fine, i feed them 3 times a day(all the flakes r gone in under 1 min)
i change the water at leat once every...