skinny fish

[mybaboo]

Has anyone else had a problem with fish gradually becoming skinnier and skinnier?
I have mostly livebearers. The PH as well as all the other water perameters are great. The fish eat and swim but just become thin and eventually die. Thier diet is healthy as well but I suspect internal parasites, but am unsure of what medication I can use to remedy this. If anyone has has success with any medications and internal parasites, it would be greatly appreciated

 

[Tanked]

More specific information would be good before medication should be recomended.
what are the ph, ammonia, nitrite, and nitrate, levels
what is the water temp?
how long has the tank been setup?
how many and what types of fish are in the tank?
are they poohing long white strands? if so then it is more than likely internal parasites. I lost a few guppies to that.