Platy Has Darkening Insides Help

I have a panda platy who seems to have darkening insides, like the stomach area. Just a week ago I had a guppy get bloated with a darkening belly and then die. I'm trying to seek advice if it's a bacterial sickness or worst. Thanks in advance.

 

Can you get a pic?

 

It's barely showing but this is how it's gotten for the past week

 

Pretty fish but I don’t really see anything.

 

Looks kinda skinny.

 

Is it young?

 

Yeah that's the problem it is getting skinny,but she eats. That little black line inside her (behind the gills) Is what's getting bigger and darker. I'm not sure her age but I've only had her a few months.

The guppy I had started acting lethargic, then didn't eat then bloated one day and died the next morning.

This platy acted lethargic but now it's bouncing back but whatever is inside her is still there and sometimes she does like a seizure every now and then at the top of the tank