sorry to hear about your loss
Ates is pretty close to boardline toxic... maybe try doing smaller, more frequent water changes?
Maybe some parasite or baceterial infection is effecting your live bearers, or perhaps you have a bad mix of males and females.
Do you keep salt in the tank?
What temperature is the water on average?
How many males do you have, and females?
I've got a ton of mollies, and several platies ... that I can remember, I only lost a single red platy about 8 months ago ... she went on a hunger strike and slowly wasted away ... it was the darndest thing