While culling fry and young guppies may seem a bit cruel its a necessary part of breeding for a specific strain.   Having an inferior fish involved in breeding sets back the whole program.  Also when you have large numbers of fry at some point it just becomes impossible to have enough room to properly care for them.   For example I have a batch of fry atleast once a week.  I've never managed to sell off guppies to the LFS at less than 3 months of age.  So if I have 20 new fry a week for 3 months that would be 240 guppies on hand at any given time.  Plus the normal breeding stock and the random drop from the guppies in the show tank.   Now to breed effectively I have to isolate the males from the females for most of the time except for when they are very very young.   So obviously it would take a huge number of tanks and time to deal with about 300 guppies at any given time.  Instead I try to keep my breeding lines at around 20 guppies, maybe 5-10 for the show tank, no more than 50 small fry at a time, and only 30 or so in the grow out tanks.   So out of a rough base of 300 guppies  I cull almost 200 just to keep things under control.  For example my last few drops of fry that are into the grow out tanks now were culled drastically.  Of the approximately 40 dark bodied guppies that started out I culled all but 10.  Of the approximately 40 yellow bodied guppies only 20 are still around.  And theres still another round of culling to go with that group to decide who gets to stay here and who is visiting the LFS!
"Man hates those to whom he feels the need to lie."