Sick? Stressed? Ich? Molly

[Alx25]

I got a fish a few days ago all ammonia level and nitrite is good and so is nitrate however one of my mollies has a white dryisish spot near its tail here it is 

 

[fluttermoth]

What are the exact numbers from your ammonia and nitrite tests and what are you using to test them? Have you tested the nitrate and pH levels as well and, if so, what were the numbers from those?
 
How big is the tank, how long has it been set up, is it cycled and what other fish (species and numbers) are in there?
 
Sorry for all the questions, but the more information we have the better we can help you and your fish