Female Mosaic Guppy Fell Sick After Gave Birth

After posting on the guppy fries, the female mosaic adult guppy fell sick!

Symptoms:
Listless, heavy breathing in gills, always lying low on bed of fish tank.
pics as below.

The parameters are as such:

1. GH General Hardness = 30 ppm, mg/L
2. KH Carbonate Hardness = between 0 to 40 ppm, mg/L
3. pH = 6.5
4. NO2 Nitrite = 0.5 ppm, mg/L
5. NO3 Nitrate = 40 ppm, mg/L

Results based on API 5 in 1 Aquarium Test Strips