Platy fry's growth?

ok i have a fish tank thats around 25 gallons (weird shape) its been up for about 7 montsh now after i cleaned it and its doing really well. Currently i have 2 platys and 2 fish i dont know the name of :). Well about a month ago my friends platy had fry the over took her tank so i took 4 from her. 3 of them died in the first week but one survied! and has been still doing good for a month now. But it seems lik him/her is now growing! I mean i know he has grown some cuasehe has got all his/her color now. But not much size.. i want him/her to grow up so it can have fry with my orange/black platy :). Why wont it grow! My platy is pregnant for the first time too and i dont wnt the same thing to happen to her fry!
Answers:
Factors that affect growth rates are ,cleanliness of the water:quality of food; and temperature.
Change 20 to 25 % of the water once or twice a week.
Keep the filters clean.
Try to find some forms of live foods,both plant and animal.
Keep the tank temperature at 78 or 79 degrees F.
All this should begin as soon as possible after the fry are dropped.
The fact that three out of four fry died makes me believe that you do not do frequent water changes and that the Nitrates and possibly also Ammonia numbers are high for your tank.
Also you should try to find out the correct name of your other fish,it's important to know what sort of fish you are trying to take care of.