Sorry to hear that you have lost some fish.
It is very upsetting, especially after the joy of finding fry in your tank.
It can sometimes make us wonder what we are doing wrong, and whether it is worth putting so much effort into setting up and maintaining a suitable environment if this is what is going to happen.
Some fish are more vulnerable to problems that others, as Sue has mentioned.
And as hampalong has pointed out, these things happen to us all occasionally. I have certainly had a few incidents where we have struggled to get to the bottom of the problem, which can be very frustrating.
Keep monitoring your tank, keep watching your fish, keep us posted.
The feelings of distress and disappointment will pass, and you will enjoy your tank again.
Keep at it.