When I was younger I had a 10g and new nothing about it. I kept guppies and had over a 100 fry at any time. I was horrendous when it came to knowing what I was doing. Not knowing anything about water quality or cycling. Finally my tank went to pieces and I got behind on cleaning. It got really nasty and that one guppy was still alive. I know better now but have always considered guppies to be really hardy. I had gotten them from a local (small) pet store. I would question the stocker. Also, everytime I go to Petco their guppies do not look well (drab colors, not that active). I will probably buy other fish from them but their guppy selection is never good. My local Petsmart has a much better guppy stock than Petco though I don't know if thats just around here.