I don't understand? Why cull a small spawn? Isn't there just as likely to be that one fish you're looking for if you were to pull any random 50 fry from this pair?
And aren't small spawns easier to deal with?
I know a lot of folks pull out the biggest ones at a month and then cull the rest just so they don't have as many to deal with - you already don't have as many to deal with, so they won't be as crowded in the tank, not so much growth hormone, not so much competition for food, etc. All the reasons folks give for culling down to about 50 in a month or so.
Not trying to judge or anything. I'm just new at this and wondering about the reasoning.