Jaime

I have had 6 Kuhli loaches in a 20 gallon for several months and they have all grown however one remained quite skinny. She started floating at the top and upside, struggling to get into the guppy grass to right herself. I have treated with dewormer, but it's been 4 days and now it seems she has given up and is just floating aimlessly at the top of the water, upside down, not hanging to the plants. Do you know what could be wrong?