There are many factors which may affect growth in fry. The most important is proper diet, in my opinion. A varied diet in sufficient amounts is critical for proper growth. I like to feed a good powdered growth food by Tetra Fry food "L" for livebearers as a staple. I also add live baby brine shrimp to the diet once a day, Frozen baby brine shrimp also works, as guppy fry don't seem to have much trouble accepting it. Crushed growth food when the fry are a little larger is also beneficial. I feed 3 to 4 times a day.
I clean the tank often, changing about 20% of the water everyday, or 25%-30% every other day, depending on my schedule. I make sure the tank is large enough, as the fry will need enough swimming space to grow.
Provide a clean cycled tank with proper and adequate filtration. Proper temperature and water parameters are also important.
With proper care, guppy fry should reach adulthood in about 8 weeks and start reproducing fry of their own, even though they may not be full size yet. Guppy fry should reach full size in about 2 1/2 months, or thereabouts.
One final note which isn't really a pleasant subject, but I believe needs to be considered, is culling. I cull ruthlessly. If a baby guppy is deformed or starts behaving erratically, stops eating, stops growing, etc...I take it out and feed it to my larger fishes. I don't really want to devote space, time, and resources to fishes that are deformed and/or are suffering. As a result, your other fishes will have a better chance at surviving..."Survival of the fittest" as they say.
