Concave guppies belly...?

[Sweet Angel]

Guppies lovers,
I've been keeping guppies more than a year now. I notice some kind of disease (found in female guppies specially), it makes fish movement slower / more sluggish, their tail swing slower when swimming, & their belly became slightly concave. Although they still eat greedily, but their belly can't go back to normal round shape again.
There's no sign of any cotton wool thingy, white spots, or fray fin, but I think their colour fade away (I kept albino strain guppies, the one with this disease tend to have more whitish in colour)
Maybe this disease spreading, but slowly (maybe depends on water volume / tank size).
Cause I had 1 female showing this sign of disease, first I kept it alone in nano betta tank, than I moved it into 4ft tank along with many other guppies (most of them are male). In that time no other guppies infected.
After about 2 months (this female ever gave birth in that 4ft tank) I moved it again to 40cm female only tank.
I don't observe this tank routinely because it has no light so I can only watch them on Sunday or holiday. Maybe after 1 month I realized that another female got this sign of disease too (the biggest female in that tank), and the carrier female became thinner than before.
Does anyone know what disease is this?
How do we prevent it, & how do we cure it...?

Tks...

[Sweet Angel]

Hi guys,

After googling found this one : http://www.fullredguppy.com/medicatedfood.html

Could it be wasting disease like mention in above link...?
Anyone here ever heard or try the medicated food...?