Sick platy

[Shleena]

My black platy had slowly been getting thinner and thinner. I originally had two of them but one had the same thing happen to it and it died. I'm not sure what to do or what disease it has but it's only my black ones and my other platy is healthy.
They live in a 20 gallon tank with 5 neon tetras 1 Molly and 1 other platy and 2 bottom feeder cat fish and a plecostamus

 

[jlk]

Welcome to AA!! Can you provide some more info so we can better help you? Exact numbers for ammonia, nitrite, nitrate and ph? What your using to test? Water change schedule? How long you have had the fish in question? Any other symptoms (not eating, strange poop, discoloration, etc)? How long the tank has been setup? Thanks!