My experience with this is that it's a true experiment in Darwinism - survival of the fittest. I used to just let my guppy tanks go natural ie. I didn't put females in breeding boxes to try and save fry, but I didn't remove any fry, either. I figured that the adults would eat most of the fry, and only the strongest, fastest ones would survive. This is true, in the beginning. Eventually, however, you end up with so many females who are giving birth within any given few day span, that there are more fry than the adults can eat. My personal experience is that as the strongest fry grow and start to use the space/oxygen/food in the tank, the weaker fry and adults die to make room. It's not at all pleasant to find dead or dying guppies every day, so the 'natural' thing wore thin with me. You have at least a couple of options. If you have other fish, they'll gladly help with population control. If this seems cruel or unappealing to you, you can start taking the juveniles to your lfs (if they'll take them). When I say juveniles, I don't mean tiny fry, but young guppies already showing colour and easily sexed. Alternatively, you can start setting up new tanks every few weeks to accomodate your guppies!