Yellow female molly with a drooping tail

[KaZ38]

My Female molly has a drooping tail. She is not using it and is slowly facing upwards. She is eating but is slower than normal probably due to not using the tail. She is now getting picked on by a small guppy, I have just lost a sharpae tetra and my black molly is sulking. I have two mollies, a guppy, gourami, cory and now 3 tetras, in a 54 litre tank which I do water changes every week.

[F15hguy]

have you had your water tested?, mollies are quite sensitive to nitrates.