pH shock/water condition shock. I think it's quite normal for this to happen if you rush things too quickly i.e. transfering your fries into another environment. This will be stressful for them so they died out of shock. My experience is never transfer your fries at least 1 week after birth. Transfer the adults instead but I just left them to mingled together. My 1st two fries died the same way because of my nosy siblings transfered it and done everything i.e putting dechlorinator+++. But I have now 2 batches up to 30 fries in the same tank with adult guppies and they are growing healthily. I only transferred them after 2 weeks into the indoor tank from the outdoor basin which was used to plant water lily because of deteriorating water quality. I have 100% survival rate of the 2 batches up to now. Hehe better luck next time