Newborn fish some have red sac under belly?

Yes it means they were premature. Something must of stressed your fish to make her have them too soon, they should be fine but if you moved the female to one of those little breeders boxes don't do that next time and just get more plants for the fry to hid in. If they aren't too premature the sac could be gone within hours but it could take a couple days, until then they don't need to be fed.

How come you've chilly water fish with tropical fish? the newborn piranha's will quickly end off the different fish you've contained in the tank & may even turn on one yet another,survival of the fittest. I actually have 5 man or woman piranha's in a 5ft tank with a huge shoal of Neons & up to now after 3years the neons nonetheless live on because the piranhas are properly fed, if goldfish were contained in the tank as well they could quickly be eaten via goldfishes colouration and so on.

They should all be born with a yolk sack. They live off this for a bit, and if no other food is available. Maybe it had been gone or depleted when you noticed your other fry?

If they are swimming normally,eating well, and water perimeters are ideal, I wouldn't worry.

The tummies will also be red when they have eaten and have a full tummy of baby brine shrimp.

the red sac is just what they eat for the first few days they will be perfectly fine .. hakuna matata.. no worries!!

A newly hatched fry uses the yolk sac as a food source!!

if the other fish were antaganizing her the she will probably be stresd out and that can lead to the fry being born prematurely. also they should do fine if you feed them properfood and are in clean water.

It is just the yolk sack. They will feed off of that for the first few days just like chicks. They will be fine : )

Maybe it means they are (fe)male?