Platies Fraying Tail Fin

[ultimateginger]

I recently started a 10 gallon freshwater tank.  After the water tests all came out good I got several live plants and 5 fish (all platies, 3 tiger platies and 2 high finned platies) and introduced them to the tank.  The fish were added two days ago and yesterday I noticed two of the fish began to have white stuff appear on their tail fins.  One other fish in particular seemed to be bullying these two fish so he was removed and put into a separate tank by himself.  Throughout the day today the two fish with the white tails appear to be getting worse, even with the third removed (they were all males so I figured it was competition).  Their tails have begun fraying, and one seems to be missing almost half his tail now.  It is only their tail fins though.  They do not have any other white spots on them and swim around like normal, they don't lay on the bottom or anything.  I gave the tank a dose of melafix today in the hopes that will help as well as some added aquarium salt.  Any idea what the problem is? Or any solutions?
Thank you for your time.

 

[PrairieSunflower]

You said you recently started your freshwater tank.  What do you mean by recently?  Did you cycle your tank before adding fish, if yes, how did you do this?
 
If you did not properly cycle you tank, then these conditions could well be caused by poor water quality.  A picture would also help to diagnose.  So far it sounds like you are describing fungus and maybe fin rot as well.

 

[the_lock_man]

I'm making a few assumptions here, but I reckon Prairie is right. I believe your filter has not got sufficient bacteria to filter out the ammonia produced by the fish, and the problem is being caused by that ammonia.
 
Have a read of the green link in my signature area, it provides the solution to your problem.