HELP-Dwarf Gourami turning gray

Feb 18, 2018
I got a dwarf gourami and she was fine the first few days I had her and eating fine but then she started turning this grayish black on her face mouth back and tail. She’s been laying on the bottom and not eating and when she would try to swim up it seemed like her tail was weighing her down. Now she just lays stil most of the time and i really don’t want her to die. I use water conditioner for the tank to make the water i put in safe it’s about 5-10 gallon tank she’s the only fish in it besides a small plecco. HELP!