female swordtails keep dying

[Misheru]

Hello everyone,

I would appreciate if it someone could tell me why our female swordtails keep dying. We've had six die already, in total. The males, who were bought first, are fine.

The females will stay on the bottom of the tank and breath heavily. They won't move even when another fish comes close to them, or when the net comes close to them. There seems to be no marks on them of any kind. They exhibit this behavior for about a day, then die. Could they be dying in childbirth? Or something... I just don't get it, there appears to be no reason for them to die. Everyone else is fine.

The ammonia is zero and the PH is normal. It's a 20 gal tank with corys, a platy and swords.

Thank you for your time.