It depends what you want to do with your fish. If your Endlers were pure ( well as pure as you can get LOL) then if the F's have been impregnanted by a guppy, she can store sperm and even if you remove her from the guppy males she may still have hybrid fry for 2 or 3 drops. The fry she has ( and any fry from EndlerM and GuppyF) will all be hybrid and their colouring not nearly so good. However you may not mind - its a bit late if you do though. If the Endler female/s have been in the tank with guppies they will almost certainly have bred already.
You can never be sure to get the pure strain back, though if you can seperate the Endlers out now and let any F's have their fry ( put them in a breeder net so there is no way they can mature and impregnate the adult F Endlersin a few eeks time) and then keep the F Endlers with Endler males, you may get 100% Endler fry in the future via sperm replacement. If you can set up another tank, put your Endler males in it and aquire some more Endler females, then all is not lost, if you want to try to keep "pure" Endlers.
PS
When I speak of "Pure" ELB's I am just using a term that to me, means that they are as close to the original wild strains as you can get, I am aware that to use the word Pure may be misleading as we just dont know which strains of Endlers have guppy close up in their ancestry. John Endler stated that in the wild, when he collected in the 70's that Endlers did not interbred with guppies, but it will happen in captivity and the more guppy genes an "Endler" carries, the more likley it is to interbreed again with a guppy.
