What Causes........

[JonesyJ666]

The rear tail/fine of a fish to droop downwards?

 

[Chad]

I'm not quite sure I know what you are asking. If it's a malformation it is often due to malnutrition, stunting,  or a genetic defect.

 

[JonesyJ666]

Well ive had this white molly for months, now she like all the others had a straightish back from mouth to tail.

This white molly now has a curved back witch is now making her tail droop down

 

[KrystaK]

Sounds like it may be stunting. How big is the tank and how many fish do you have in it? 

 

[JonesyJ666]

its 6ft x 2ft x 2ft and about 10 fish

 

[Blobfish]

I've had some fish which developed a humped back as they got to the end of their lives, the last one been a Galaxy Rasbora which I had for around 3 years.  In the month before he passed on he developed a curved spine, stopped eating and generally seemed unwell.