illness

Question
what to do in fish feather torn disease..and why same fish bite her fellow

Answer
Hello Nabil,
When fish have ragged fins, it is fin, and tail rot.  This is due to poor water conditions.  Check you water to make sure you have no ammonia, or nitrates in it.  If you do have ammonia, or nitrates, make water changes every day of 15% to get rid of it.  Once your water chemistry is okay, you will have to treat with an antibiotic.  "Kanamycin"  Tetracycline" or "Furan 2" are good medication to use.  Follow the directions on the package.  Always keep your water clean, and do water changes every week of 25% using a good water conditioner.
Some fish will attack their mate after the spawn.  A female will sometimes viciously chase him away.  Some fish get along, some don't.  I do not know what fish you have, or the size of your tank.  Some fish need lots of space, and hiding places.  If they do not have the space, or the hiding places, they will fight, even with their own kind.  You will have to separate them, as they will continue to fight.  Sometimes adding more rocks, hiding places, helps.
I hope this helps, and that I have answered your question.
Lynda