Platy Fry Mass Deaths

We had bought a very pregnant female Platy and when she finally had them 1 long week later we manage to find MOST of the babies(she had to have had a good 30 or so). She had had them before we could get her in the breeder and the breeder we have was a little to small for how big she had gotten.
Well the fry were ok for the 1st few days in the breeder but our of no where after those few days we found a good portion of her fry had died. Now they are in a tank with some Guppy fry and all we can find is about 5 (I'm sure some may just be good at hiding).

I was just curious if anyone knew what could have caused so many to die in such a short period of time ?

When you switched the fry from 1 tank to another do these 2 tanks have the same parameters as far as pH and hardness?
How many times a day have you fed them?

i guess i should have phrased that better. we didn't switch tanks just released them from the breeder into the tank itself after we took the female out.

The stress of buying the female may have caused her to give birth early perhaps causing the babies to be premature...?

How long have you had the tank running and cycled?

it was running for a couple weeks because we had some of the guppies in there before hand. I don't think it was stress from buying the female pregnant, I'd imagine if it was stress she would have had them before we got her home or soon after we got her in the tank.

could it have been overcrowded ?
those breeders arent all that big but them themselves aren't either.

posted in wrong spot. Sorry!