Aquarium Fishes

Question
Sir,
I have a tank of approx. 150gal. Keeping only Platty, Sword Tail, Molly and Guppies. Tank is having natural plants and all kind of the fish are breeding. Happy with the setup.
But sometimes suddenly they start dying, the symptom is the shape of the fish becomes flattened, stomach is shrinking and become weak and after few days they die.
Waht could be the reason and how to prevent and cure ?
Also sometimes the fish looks like it is trying to swim fast fast but not moving !!
Regards,
Philip

Answer
Hello,
The shriveled up flat stomaches sound like what happens after they give birth, with that and the added stress of being chased by males will kill them, and the trying to swim but not moving sound like there giving birth OR your filters too strong. for pregnant females, i would remove them when there close to giving birth or after giving birth so they can have a week or so to regain stregnth.